Skip to content
Biome
Docs Blog Playground

useConst (since v1.0.0)

Diagnostic Category: lint/style/useConst

Sources:

Require const declarations for variables that are only assigned once.

Variables that are initialized and never reassigned and variables that are only assigned once can be declared as const.

Examples

Section titled Examples

Invalid

Section titled Invalid
let a = 3;
console.log(a);
style/useConst.js:1:1 lint/style/useConst  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This let declares a variable that is only assigned once.
  
  > 1 │ let a = 3;
      │ ^^^
    2 │ console.log(a);
    3 │ 
  
  ℹ 'a' is never reassigned.
  
  > 1 │ let a = 3;
      │     ^
    2 │ console.log(a);
    3 │ 
  
  ℹ Safe fix: Use const instead.
  
    1   │ - let·a·=·3;
      1 │ + const·a·=·3;
    2 2 │   console.log(a);
    3 3 │   
  

// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
    console.log(a);
}
style/useConst.js:2:6 lint/style/useConst  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This let declares a variable that is only assigned once.
  
    1 │ // `a` is redefined (not reassigned) on each loop step.
  > 2 │ for (let a of [1, 2, 3]) {
      │      ^^^
    3 │     console.log(a);
    4 │ }
  
  ℹ 'a' is never reassigned.
  
    1 │ // `a` is redefined (not reassigned) on each loop step.
  > 2 │ for (let a of [1, 2, 3]) {
      │          ^
    3 │     console.log(a);
    4 │ }
  
  ℹ Safe fix: Use const instead.
  
    1 1 │   // `a` is redefined (not reassigned) on each loop step.
    2   │ - for·(let·a·of·[1,·2,·3])·{
      2 │ + for·(const·a·of·[1,·2,·3])·{
    3 3 │       console.log(a);
    4 4 │   }
  

// `a` is redefined (not reassigned) on each loop step.
for (let a in [1, 2, 3]) {
    console.log(a);
}
style/useConst.js:2:6 lint/style/useConst  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This let declares a variable that is only assigned once.
  
    1 │ // `a` is redefined (not reassigned) on each loop step.
  > 2 │ for (let a in [1, 2, 3]) {
      │      ^^^
    3 │     console.log(a);
    4 │ }
  
  ℹ 'a' is never reassigned.
  
    1 │ // `a` is redefined (not reassigned) on each loop step.
  > 2 │ for (let a in [1, 2, 3]) {
      │          ^
    3 │     console.log(a);
    4 │ }
  
  ℹ Safe fix: Use const instead.
  
    1 1 │   // `a` is redefined (not reassigned) on each loop step.
    2   │ - for·(let·a·in·[1,·2,·3])·{
      2 │ + for·(const·a·in·[1,·2,·3])·{
    3 3 │       console.log(a);
    4 4 │   }
  

let a;
a = 0;
style/useConst.js:1:1 lint/style/useConst ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This let declares a variable that is only assigned once.
  
  > 1 │ let a;
      │ ^^^
    2 │ a = 0;
    3 │ 
  
  ℹ 'a' is only assigned here.
  
    1 │ let a;
  > 2 │ a = 0;
      │ ^
    3 │ 
  

let a = 3;
{
    let a = 4;
    a = 2;
}
style/useConst.js:1:1 lint/style/useConst  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This let declares a variable that is only assigned once.
  
  > 1 │ let a = 3;
      │ ^^^
    2 │ {
    3 │     let a = 4;
  
  ℹ 'a' is never reassigned.
  
  > 1 │ let a = 3;
      │     ^
    2 │ {
    3 │     let a = 4;
  
  ℹ Safe fix: Use const instead.
  
    1   │ - let·a·=·3;
      1 │ + const·a·=·3;
    2 2 │   {
    3 3 │       let a = 4;

Valid

Section titled Valid
let a = 2;
a = 3;
console.log(a);
let a = 1, b = 2;
b = 3;
let a;
a; // the variable is read before its assignement
a = 0;
Section titled Related links