ReferenceError: can't access lexical declaration`X' before initialization

我們的志工尚未將本文翻譯為 正體中文 (繁體) 版本。加入我們,幫忙翻譯!
您也可以閱讀本文的 English (US) 版本。

Message

ReferenceError: can't access lexical declaration `X' before initialization (Firefox)
ReferenceError: 'x' is not defined (Chrome)

Error type

ReferenceError

What went wrong?

A lexical variable was accessed before it was initialized. This happens within any block statement, when let or const declarations are accessed before they are defined.

Examples

Invalid cases

In this case, the variable "foo" is redeclared in the block statement using let.

function test() {
  let foo = 33;
  if (true) {
    let foo = (foo + 55); 
    // ReferenceError: can't access lexical 
    // declaration `foo' before initialization
  }
}
test();

Valid cases

To change "foo" inside the if statement, you need to remove the let that causes the redeclaration.

function test(){
   let foo = 33;
   if (true) {
      foo = (foo + 55);
   }
}
test();

See also

文件標籤與貢獻者

 此頁面的貢獻者: fscholz, johnjago, jonathanKingston
 最近更新: fscholz,