useIndexOf

JavaScript (and super languages)

Summary

• Rule available since: v2.0.0
• Diagnostic Category: lint/complexity/useIndexOf
• This rule is recommended, which means is enabled by default.
• This rule has an unsafe fix.
• The default severity of this rule is information.
• Sources:
  • Same as unicorn/prefer-array-index-of

How to configure

biome.json

{
  "linter": {
    "rules": {
      "complexity": {
        "useIndexOf": "error"
      }
    }
  }
}

Description

Prefer Array#{indexOf,lastIndexOf}() over Array#{findIndex,findLastIndex}() when looking for the index of an item.

Array#findIndex() and Array#findLastIndex() are intended for more complex needs. If you are just looking for the index where the given item is present, then the code can be simplified to use Array#indexOf() or Array#lastIndexOf(). This applies to any search with a literal, a variable, or any expression that doesn’t have any explicit side effects. However, if the expression you are looking for relies on an item related to the function (its arguments, the function self, etc.), the case is still valid.

Examples

Invalid

const index = foo.findIndex(x => x === 'foo');
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findIndex(x =&gt; x === 'foo');<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>indexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">'</span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

const index = foo.findIndex(x => 'foo' === x);
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findIndex(x =&gt; 'foo' === x);<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>indexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

const index = foo.findIndex(x => {
     return x === 'foo';
});
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findIndex(x =&gt; &#123;<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>     return x === 'foo';<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>3 │ </strong>&#125;);<br />   <strong>   │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>4 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>indexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#indexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>&#123;</strong></span><br />    <strong>2</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><strong>;</strong></span><br />    <strong>3</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><strong>&#125;</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>i</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>4</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

const index = foo.findLastIndex(x => 'foo' === x);
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findLastIndex(x =&gt; 'foo' === x);<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>2 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>lastIndexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>L</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>s</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>s</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;"><strong>I</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>2</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

const index = foo.findLastIndex(x => {
     return x === 'bar';
});
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findLastIndex(x =&gt; &#123;<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>     return x === 'bar';<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>3 │ </strong>&#125;);<br />   <strong>   │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>4 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>lastIndexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>L</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>s</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>&gt;</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>&#123;</strong></span><br />    <strong>2</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">b</span><span style="color: Tomato;">a</span><span style="color: Tomato;">r</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><strong>;</strong></span><br />    <strong>3</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><strong>&#125;</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>s</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;"><strong>I</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">b</span><span style="color: MediumSeaGreen;">a</span><span style="color: MediumSeaGreen;">r</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>4</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

const index = foo.findLastIndex(function(x) {
     return x === 'foo';
});
<pre class="language-text"><code class="language-text">code-block.js:1:15 <a href="https://biomejs.dev/linter/rules/use-index-of">lint/complexity/useIndexOf</a> <span style="color: #000; background-color: #ddd;"> FIXABLE </span> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Prefer </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><span style="color: lightgreen;"> over </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> when looking for the index of an item.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>const index = foo.findLastIndex(function(x) &#123;<br />   <strong>   │ </strong>              <strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>2 │ </strong>     return x === 'foo';<br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>3 │ </strong>&#125;);<br />   <strong>   │ </strong><strong><span style="color: Tomato;">^</span></strong><strong><span style="color: Tomato;">^</span></strong><br />    <strong>4 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">This callback only tests for equality against a single value. This value can be passed directly to </span><span style="color: lightgreen;"><strong>lastIndexOf()</strong></span><span style="color: lightgreen;"> instead.</span><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Unsafe fix</span><span style="color: lightgreen;">: </span><span style="color: lightgreen;">Replace </span><span style="color: lightgreen;"><strong>Array#findLastIndex()</strong></span><span style="color: lightgreen;"> with </span><span style="color: lightgreen;"><strong>Array#lastIndexOf()</strong></span><br />  <br />    <strong>1</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;">c</span><span style="color: Tomato;">o</span><span style="color: Tomato;">n</span><span style="color: Tomato;">s</span><span style="color: Tomato;">t</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">i</span><span style="color: Tomato;">n</span><span style="color: Tomato;">d</span><span style="color: Tomato;">e</span><span style="color: Tomato;">x</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">=</span><span style="color: Tomato;"><span style="opacity: 0.8;">·</span></span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">.</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>L</strong></span><span style="color: Tomato;"><strong>a</strong></span><span style="color: Tomato;"><strong>s</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>I</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>d</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;">(</span><span style="color: Tomato;"><strong>f</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>c</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>i</strong></span><span style="color: Tomato;"><strong>o</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><strong>(</strong></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><strong>)</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>&#123;</strong></span><br />    <strong>2</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>e</strong></span><span style="color: Tomato;"><strong>t</strong></span><span style="color: Tomato;"><strong>u</strong></span><span style="color: Tomato;"><strong>r</strong></span><span style="color: Tomato;"><strong>n</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>x</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><strong>=</strong></span><span style="color: Tomato;"><span style="opacity: 0.8;"><strong>·</strong></span></span><span style="color: Tomato;">'</span><span style="color: Tomato;">f</span><span style="color: Tomato;">o</span><span style="color: Tomato;">o</span><span style="color: Tomato;">'</span><span style="color: Tomato;"><strong>;</strong></span><br />    <strong>3</strong>  <strong> │ </strong><span style="color: Tomato;">-</span> <span style="color: Tomato;"><strong>&#125;</strong></span><span style="color: Tomato;">)</span><span style="color: Tomato;">;</span><br />      <strong>1</strong><strong> │ </strong><span style="color: MediumSeaGreen;">+</span> <span style="color: MediumSeaGreen;">c</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">s</span><span style="color: MediumSeaGreen;">t</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">i</span><span style="color: MediumSeaGreen;">n</span><span style="color: MediumSeaGreen;">d</span><span style="color: MediumSeaGreen;">e</span><span style="color: MediumSeaGreen;">x</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">=</span><span style="color: MediumSeaGreen;"><span style="opacity: 0.8;">·</span></span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">.</span><span style="color: MediumSeaGreen;"><strong>l</strong></span><span style="color: MediumSeaGreen;"><strong>a</strong></span><span style="color: MediumSeaGreen;"><strong>s</strong></span><span style="color: MediumSeaGreen;"><strong>t</strong></span><span style="color: MediumSeaGreen;"><strong>I</strong></span><span style="color: MediumSeaGreen;"><strong>n</strong></span><span style="color: MediumSeaGreen;"><strong>d</strong></span><span style="color: MediumSeaGreen;"><strong>e</strong></span><span style="color: MediumSeaGreen;"><strong>x</strong></span><span style="color: MediumSeaGreen;"><strong>O</strong></span><span style="color: MediumSeaGreen;"><strong>f</strong></span><span style="color: MediumSeaGreen;">(</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">f</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">o</span><span style="color: MediumSeaGreen;">'</span><span style="color: MediumSeaGreen;">)</span><span style="color: MediumSeaGreen;">;</span><br />    <strong>4</strong> <strong>2</strong><strong> │ </strong>  <br />  <br /></code></pre>

Valid

const index = foo.indexOf('foo');

const index = foo.findIndex(x => x !== 'foo');

const index = foo.findIndex((x, index) => x === index);

const index = foo.findIndex(x => (x === 'foo') && isValid());

const index = foo.findIndex(x => y === 'foo');

const index = foo.findIndex(x => y.x === 'foo');

const index = foo.findIndex(x => {
    const bar = getBar();
    return x === bar;
});

const index = foo.findIndex(function(x) {
    const bar = getBar();
    return x === bar;
});

const index = foo.lastIndexOf('foo');

const index = foo.findLastIndex(x => x !== 'foo');

const index = foo.findLastIndex((x, index) => x === index);

const index = foo.findLastIndex(x => (x === 'foo') && isValid());

const index = foo.findLastIndex(x => y === 'foo');

const index = foo.findLastIndex(x => y.x === 'foo');

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