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noInvalidUseBeforeDeclaration (since v1.5.0)

Diagnostic Category: lint/correctness/noInvalidUseBeforeDeclaration

Sources:

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

Section titled Examples

Invalid

Section titled Invalid
function f() {
    console.log(x);
    const x;
}
correctness/noInvalidUseBeforeDeclaration.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.
  

function f() {
    console.log(x);
    var x = 0;
}
correctness/noInvalidUseBeforeDeclaration.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 │ 
  

function f(a = b, b = 0) {}
correctness/noInvalidUseBeforeDeclaration.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

Section titled Valid
f();
function f() {}


new C();
class C {}
// An export can reference a variable before its declaration.
export { CONSTANT };
const CONSTANT = 0;
function f() { return CONSTANT; }
const CONSTANT = 0;
Section titled Related links