noUnassignedVariables

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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

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

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

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let status;
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if (status === 'ready') {
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    console.log('Status is ready');
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}
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<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>

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

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let message = "hello";
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console.log(message);
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let user;
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user = getUser();
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console.log(user.name);
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let count;
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count = 0;
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count++;

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declare let value: number | undefined;
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console.log(value);
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declare module "my-module" {
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    let value: string;
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    export = value;
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}