noInvalidUseBeforeDeclaration

诊断类别：lint/correctness/noInvalidUseBeforeDeclaration

¥Diagnostic Category: lint/correctness/noInvalidUseBeforeDeclaration

JavaScript (and super languages)

自从：v1.5.0

¥Since: v1.5.0

来源：

¥Sources:

与以下相同：no-use-before-define

¥Same as: no-use-before-define
与以下相同：@typescript-eslint/no-use-before-define

¥Same as: @typescript-eslint/no-use-before-define

禁止在声明之前使用变量和函数参数

¥Disallow the use of variables and function parameters before their declaration

JavaScript 不允许在声明之前使用块范围变量（let、const）和函数参数。在声明变量或参数之前，任何尝试访问该变量或参数都会抛出 ReferenceError。

¥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.

该规则还报告在声明之前使用 var 声明的变量的使用情况。

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

示例

¥Examples

无效

¥Invalid

function f() {
    console.log(x);
    const x;
}

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.

function f() {
    console.log(x);
    var x = 0;
}

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 │

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 │

有效

¥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;

noInvalidUseBeforeDeclaration

示例

无效

有效

相关链接