noRedeclare (JavaScript)

Language JavaScript (and super languages)

Summary

• Rule available since: v1.0.0
• Diagnostic Category: lint/suspicious/noRedeclare
• This rule is recommended, meaning it is enabled by default.
• This rule doesn’t have a fix.
• The default severity of this rule is error.
• Sources:
  • Same as no-redeclare
  • Same as @typescript-eslint/no-redeclare

How to configure

biome.json

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

Description

Disallow variable, function, class, and type redeclarations in the same scope.

Examples

Invalid

var a = 3;
var a = 10;

code-block.js:2:5 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ 'a' is redeclared in the same scope.
  
    1 │ var a = 3;
  > 2 │ var a = 10;
      │     ^
    3 │ 
  
  ℹ 'a' is defined here:
  
  > 1 │ var a = 3;
      │     ^
    2 │ var a = 10;
    3 │ 
  
  ℹ Redeclarations make it unclear which variable a reference points to and can hide earlier declarations.
  
  ℹ Remove the duplicate declaration or rename one of the variables.
  

let a = 3;
let a = 10;

code-block.js:2:5 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ 'a' is redeclared in the same scope.
  
    1 │ let a = 3;
  > 2 │ let a = 10;
      │     ^
    3 │ 
  
  ℹ 'a' is defined here:
  
  > 1 │ let a = 3;
      │     ^
    2 │ let a = 10;
    3 │ 
  
  ℹ Redeclarations make it unclear which variable a reference points to and can hide earlier declarations.
  
  ℹ Remove the duplicate declaration or rename one of the variables.
  

function f() {}
function f() {}

code-block.js:2:10 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ 'f' is redeclared in the same scope.
  
    1 │ function f() {}
  > 2 │ function f() {}
      │          ^
    3 │ 
  
  ℹ 'f' is defined here:
  
  > 1 │ function f() {}
      │          ^
    2 │ function f() {}
    3 │ 
  
  ℹ Redeclarations make it unclear which variable a reference points to and can hide earlier declarations.
  
  ℹ Remove the duplicate declaration or rename one of the variables.
  

class C {
    static {
        var c = 3;
        var c = 10;
    }
}

code-block.js:4:13 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ 'c' is redeclared in the same scope.
  
    2 │     static {
    3 │         var c = 3;
  > 4 │         var c = 10;
      │             ^
    5 │     }
    6 │ }
  
  ℹ 'c' is defined here:
  
    1 │ class C {
    2 │     static {
  > 3 │         var c = 3;
      │             ^
    4 │         var c = 10;
    5 │     }
  
  ℹ Redeclarations make it unclear which variable a reference points to and can hide earlier declarations.
  
  ℹ Remove the duplicate declaration or rename one of the variables.
  

type Person = { name: string; }
class Person { name: string; }

code-block.ts:2:7 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ 'Person' is redeclared in the same scope.
  
    1 │ type Person = { name: string; }
  > 2 │ class Person { name: string; }
      │       ^^^^^^
    3 │ 
  
  ℹ 'Person' is defined here:
  
  > 1 │ type Person = { name: string; }
      │      ^^^^^^
    2 │ class Person { name: string; }
    3 │ 
  
  ℹ Redeclarations make it unclear which variable a reference points to and can hide earlier declarations.
  
  ℹ Remove the duplicate declaration or rename one of the variables.
  

Valid

var a = 3;
a = 10;

class Foo {
    bar(a: A);
    bar(a: A, b: B);
    bar(a: A, b: B) {}
}

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