noUnassignedVariables

JavaScript (and super languages)

Summary

• Rule available since: v2.1.0
• Diagnostic Category: lint/suspicious/noUnassignedVariables
• This rule doesn’t have a fix.
• The default severity of this rule is information.
• Sources:
  • Same as no-unassigned-vars

How to configure

biome.json

{
  "linter": {
    "rules": {
      "suspicious": {
        "noUnassignedVariables": "error"
      }
    }
  }
}

Description

Disallow let or var variables that are read but never assigned.

This rule flags let or var declarations that are never assigned a value but are still read or used in the code. Since these variables will always be undefined, their usage is likely a programming mistake.

Examples

Invalid

let status;
if (status === 'ready') {
    console.log('Status is ready');
}
<pre class="language-text"><code class="language-text">code-block.js:1:5 <a href="https://biomejs.dev/linter/rules/no-unassigned-variables">lint/suspicious/noUnassignedVariables</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The variable '</span><span style="color: lightgreen;"><strong>status</strong></span><span style="color: lightgreen;">' is declared but never assigned a value.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>let status;<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><br />    <strong>2 │ </strong>if (status === 'ready') &#123;<br />    <strong>3 │ </strong>    console.log('Status is ready');<br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Variable declared without assignment. Either assign a value or remove the declaration.</span><br />  <br /></code></pre>

let value: number | undefined;
console.log(value);
<pre class="language-text"><code class="language-text">code-block.ts:1:5 <a href="https://biomejs.dev/linter/rules/no-unassigned-variables">lint/suspicious/noUnassignedVariables</a> ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━<br /><br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">The variable '</span><span style="color: lightgreen;"><strong>value</strong></span><span style="color: lightgreen;">' is declared but never assigned a value.</span><br />  <br />  <strong><span style="color: Tomato;">&gt;</span></strong> <strong>1 │ </strong>let value: number | undefined;<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><br />    <strong>2 │ </strong>console.log(value);<br />    <strong>3 │ </strong><br />  <br />  <strong><span style="color: lightgreen;">ℹ</span></strong> <span style="color: lightgreen;">Variable declared without assignment. Either assign a value or remove the declaration.</span><br />  <br /></code></pre>

Valid

let message = "hello";
console.log(message);

let user;
user = getUser();
console.log(user.name);

let count;
count = 0;
count++;

declare let value: number | undefined;
console.log(value);

declare module "my-module" {
    let value: string;
    export = value;
}

Related links

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