noUndeclaredVariables

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JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/correctness/noUndeclaredVariables
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Same as no-undef

How to configure

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUndeclaredVariables": "error"
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      }
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    }
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  }
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}

Description

Prevents the usage of variables that haven’t been declared inside the document.

If you need to allow-list some global bindings, you can use the javascript.globals configuration.

Examples

Invalid

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foobar;

code-block.js:1:1 lint/correctness/noUndeclaredVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The foobar variable is undeclared.
  
  > 1 │ foobar;
      │ ^^^^^^
    2 │ 
  
  ℹ By default, Biome recognizes browser and Node.js globals.
    You can ignore more globals using the javascript.globals configuration.

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// throw diagnostic for JavaScript files
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PromiseLike;

code-block.js:2:1 lint/correctness/noUndeclaredVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The PromiseLike variable is undeclared.
  
    1 │ // throw diagnostic for JavaScript files
  > 2 │ PromiseLike;
      │ ^^^^^^^^^^^
    3 │ 
  
  ℹ By default, Biome recognizes browser and Node.js globals.
    You can ignore more globals using the javascript.globals configuration.

Valid

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type B<T> = PromiseLike<T>

Options

`checkTypes`

When set to true, it checks for undeclared types too. The option defaults to false.

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{
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  "linter": {
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    "rules": {
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      "correctness": {
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        "noUndeclaredVariables": {
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          "options": {
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            "checkTypes": true
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          }
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        }
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      }
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    }
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  }
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}

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type A = number extends infer T ? never : T;

code-block.ts:1:43 lint/correctness/noUndeclaredVariables ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ The T variable is undeclared.
  
  > 1 │ type A = number extends infer T ? never : T;
      │                                           ^
    2 │ 
  
  ℹ By default, Biome recognizes browser and Node.js globals.
    You can ignore more globals using the javascript.globals configuration.