noShadow

JavaScript (and super languages)

Caution

This rule is part of the nursery group. This means that it is experimental and the behavior can change at any time.

Summary

• Rule available since: v2.0.0
• Diagnostic Category: lint/nursery/noShadow
• This rule doesn’t have a fix.
• The default severity of this rule is warning.
• Sources:
  • Same as no-shadow
  • Same as @typescript-eslint/no-shadow

How to configure

biome.json

{
  "linter": {
    "rules": {
      "nursery": {
        "noShadow": "error"
      }
    }
  }
}

Description

Disallow variable declarations from shadowing variables declared in the outer scope.

Shadowing is the process by which a local variable shares the same name as a variable in its containing scope. This can cause confusion while reading the code and make it impossible to access the global variable.

See also: noShadowRestrictedNames

Examples

Invalid

const foo = "bar";
if (true) {
   const foo = "baz";
}

code-block.js:3:10 lint/nursery/noShadow ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable shadows another variable with the same name in the outer scope.
  
    1 │ const foo = “bar”;
    2 │ if (true) {
  > 3 │    const foo = “baz”;
      │          ^^^
    4 │ }
    5 │ 
  
  ℹ This is the shadowed variable, which is now inaccessible in the inner scope.
  
  > 1 │ const foo = “bar”;
      │       ^^^
    2 │ if (true) {
    3 │    const foo = “baz”;
  
  ℹ Consider renaming this variable. It’s easy to confuse the origin of variables if they share the same name.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  

Variable declarations in functions can shadow variables in the outer scope:

const foo = "bar";
const bar = function () {
    const foo = 10;
}

code-block.js:3:11 lint/nursery/noShadow ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable shadows another variable with the same name in the outer scope.
  
    1 │ const foo = “bar”;
    2 │ const bar = function () {
  > 3 │     const foo = 10;
      │           ^^^
    4 │ }
    5 │ 
  
  ℹ This is the shadowed variable, which is now inaccessible in the inner scope.
  
  > 1 │ const foo = “bar”;
      │       ^^^
    2 │ const bar = function () {
    3 │     const foo = 10;
  
  ℹ Consider renaming this variable. It’s easy to confuse the origin of variables if they share the same name.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  

Function argument names can shadow variables in the outer scope:

const foo = "bar";
function bar(foo) {
    foo = 10;
}

code-block.js:2:14 lint/nursery/noShadow ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable shadows another variable with the same name in the outer scope.
  
    1 │ const foo = “bar”;
  > 2 │ function bar(foo) {
      │              ^^^
    3 │     foo = 10;
    4 │ }
  
  ℹ This is the shadowed variable, which is now inaccessible in the inner scope.
  
  > 1 │ const foo = “bar”;
      │       ^^^
    2 │ function bar(foo) {
    3 │     foo = 10;
  
  ℹ Consider renaming this variable. It’s easy to confuse the origin of variables if they share the same name.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  

Valid

const foo = "bar";
if (true) {
   const qux = "baz";
}

Options

ignoreTypeValueShadow

Default: true

When enabled, a value binding that shares its name with a type-only declaration (type alias or interface) is not flagged, since types and values occupy separate namespaces in TypeScript.

When set to false, those cases are flagged:

biome.json

{
  "linter": {
    "rules": {
      "nursery": {
        "noShadow": {
          "options": {
            "ignoreTypeValueShadow": false
          }
        }
      }
    }
  }
}

type Foo = number;
function f(Foo: string) {}

code-block.ts:2:12 lint/nursery/noShadow ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable shadows another variable with the same name in the outer scope.
  
    1 │ type Foo = number;
  > 2 │ function f(Foo: string) {}
      │            ^^^
    3 │ 
  
  ℹ This is the shadowed variable, which is now inaccessible in the inner scope.
  
  > 1 │ type Foo = number;
      │      ^^^
    2 │ function f(Foo: string) {}
    3 │ 
  
  ℹ Consider renaming this variable. It’s easy to confuse the origin of variables if they share the same name.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  

ignoreFunctionTypeParameterNameValueShadow

Default: true

When enabled, parameter names in function type annotations (e.g. (x: string) => void) can share names with outer variables without being flagged.

When set to false, those cases are flagged:

biome.json

{
  "linter": {
    "rules": {
      "nursery": {
        "noShadow": {
          "options": {
            "ignoreFunctionTypeParameterNameValueShadow": false
          }
        }
      }
    }
  }
}

const test = 1;
type Fn = (test: string) => typeof test;

code-block.ts:2:12 lint/nursery/noShadow ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ This variable shadows another variable with the same name in the outer scope.
  
    1 │ const test = 1;
  > 2 │ type Fn = (test: string) => typeof test;
      │            ^^^^
    3 │ 
  
  ℹ This is the shadowed variable, which is now inaccessible in the inner scope.
  
  > 1 │ const test = 1;
      │       ^^^^
    2 │ type Fn = (test: string) => typeof test;
    3 │ 
  
  ℹ Consider renaming this variable. It’s easy to confuse the origin of variables if they share the same name.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  

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