noConstAssign (JavaScript)

Language JavaScript (and super languages)

Summary

• Rule available since: v1.0.0
• Diagnostic Category: lint/correctness/noConstAssign
• This rule is recommended, meaning it is enabled by default.
• This rule has an unsafe fix.
• The default severity of this rule is error.
• Sources:
  • Same as no-const-assign

How to configure

biome.json

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

Description

Prevents from having const variables being re-assigned.

Trying to assign a value to a const will cause an TypeError when the code is executed.

Examples

Invalid

const a = 1;
a = 4;

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

  ✖ Can't assign a because it's a constant.
  
    1 │ const a = 1;
  > 2 │ a = 4;
      │ ^
    3 │ 
  
  ℹ This is where the variable is defined as constant.
  
  > 1 │ const a = 1;
      │       ^
    2 │ a = 4;
    3 │ 
  
  ℹ Unsafe fix: Replace const with let if you assign it to a new value.
  
    1   │ - const·a·=·1;
      1 │ + let·a·=·1;
    2 2 │   a = 4;
    3 3 │   
  

const a = 2;
a += 1;

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

  ✖ Can't assign a because it's a constant.
  
    1 │ const a = 2;
  > 2 │ a += 1;
      │ ^
    3 │ 
  
  ℹ This is where the variable is defined as constant.
  
  > 1 │ const a = 2;
      │       ^
    2 │ a += 1;
    3 │ 
  
  ℹ Unsafe fix: Replace const with let if you assign it to a new value.
  
    1   │ - const·a·=·2;
      1 │ + let·a·=·2;
    2 2 │   a += 1;
    3 3 │   
  

const a = 1;
++a;

code-block.js:2:3 lint/correctness/noConstAssign  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Can't assign a because it's a constant.
  
    1 │ const a = 1;
  > 2 │ ++a;
      │   ^
    3 │ 
  
  ℹ This is where the variable is defined as constant.
  
  > 1 │ const a = 1;
      │       ^
    2 │ ++a;
    3 │ 
  
  ℹ Unsafe fix: Replace const with let if you assign it to a new value.
  
    1   │ - const·a·=·1;
      1 │ + let·a·=·1;
    2 2 │   ++a;
    3 3 │   
  

const a = 1, b = 2;

a = 2;

code-block.js:3:1 lint/correctness/noConstAssign  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Can't assign a because it's a constant.
  
    1 │ const a = 1, b = 2;
    2 │ 
  > 3 │ a = 2;
      │ ^
    4 │ 
  
  ℹ This is where the variable is defined as constant.
  
  > 1 │ const a = 1, b = 2;
      │       ^
    2 │ 
    3 │ a = 2;
  
  ℹ Unsafe fix: Replace const with let if you assign it to a new value.
  
    1   │ - const·a·=·1,·b·=·2;
      1 │ + let·a·=·1,·b·=·2;
    2 2 │   
    3 3 │   a = 2;
  

Valid

const a = 10;
let b = 10;
b = 20;

Related links

• Disable a rule
• Configure the code fix
• Rule options
• Source Code (Edit this Page)
• Test Cases
• Try in the playground