noInvalidUseBeforeDeclaration

JavaScript (and super languages)

Summary

Rule available since: v1.5.0
Diagnostic Category: lint/correctness/noInvalidUseBeforeDeclaration
This rule is recommended, which means is enabled by default.
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Same as no-use-before-define
- Same as @typescript-eslint/no-use-before-define

Description

Disallow the use of variables and function parameters before their declaration

JavaScript doesn’t allow the use of block-scoped variables (let, const) and function parameters before their declaration. A ReferenceError will be thrown with any attempt to access the variable or the parameter before its declaration.

The rule also reports the use of variables declared with var before their declarations.

Examples

Invalid

1
function f() {
2
    console.log(x);
3
    const x;
4
}

code-block.js:3:11 parse ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Const declarations must have an initialized value.
  
    1 │ function f() {
    2 │     console.log(x);
  > 3 │     const x;
      │           ^
    4 │ }
    5 │ 
  
  ℹ This variable needs to be initialized.

1
function f() {
2
    console.log(x);
3
    var x = 0;
4
}

code-block.js:2:17 lint/correctness/noInvalidUseBeforeDeclaration ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This variable is used before its declaration.
  
    1 │ function f() {
  > 2 │     console.log(x);
      │                 ^
    3 │     var x = 0;
    4 │ }
  
  ℹ The variable is declared here:
  
    1 │ function f() {
    2 │     console.log(x);
  > 3 │     var x = 0;
      │         ^
    4 │ }
    5 │

1
function f(a = b, b = 0) {}

code-block.js:1:16 lint/correctness/noInvalidUseBeforeDeclaration ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This parameter is used before its declaration.
  
  > 1 │ function f(a = b, b = 0) {}
      │                ^
    2 │ 
  
  ℹ The parameter is declared here:
  
  > 1 │ function f(a = b, b = 0) {}
      │                   ^
    2 │

Valid

1
f();
2
function f() {}
3

4
new C();
5
class C {}

1
// An export can reference a variable before its declaration.
2
export { CONSTANT };
3
const CONSTANT = 0;

1
function f() { return CONSTANT; }
2
const CONSTANT = 0;

How to configure

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