useConst

JavaScript (and super languages)

Summary

Rule available since: v1.0.0
Diagnostic Category: lint/style/useConst
This rule has a safe fix.
The default severity of this rule is warning.
Sources:
- Same as prefer-const

Description

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

Invalid

1
let a = 3;
2
console.log(a);

code-block.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 │

1
// `a` is redefined (not reassigned) on each loop step.
2
for (let a of [1, 2, 3]) {
3
    console.log(a);
4
}

code-block.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 │   }

1
// `a` is redefined (not reassigned) on each loop step.
2
for (let a in [1, 2, 3]) {
3
    console.log(a);
4
}

code-block.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 │   }

1
let a;
2
a = 0;

code-block.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 │

1
let a = 3;
2
{
3
    let a = 4;
4
    a = 2;
5
}

code-block.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

1
let a = 2;
2
a = 3;
3
console.log(a);

1
let a = 1, b = 2;
2
b = 3;

1
let a;
2
a; // the variable is read before its assignement
3
a = 0;

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "style": {
5
        "useConst": "error"
6
      }
7
    }
8
  }
9
}

useConst

Summary

Description

Examples

Invalid

Valid

How to configure

Related links