为什么在内部范围内未定义x？ [英] Why is x undefined in inner scope?

问题描述

在下面的代码中

var x = 1;

(function () {
  console.log(x);
  var x = 2;
}());

为什么当console.log（x），x未定义时？

Why is it that when console.log(x), x is undefined?

推荐答案

可变吊装。实际代码就像这样执行。

Variable hoisting. The actual code is executed like this.

var x = 1;
(function() {
    var x; // x = undefined
    console.log(x);
    x = 2;
})();

编辑：根据Lister先生的建议，有点关于变量提升。来自MDN（ https://developer.mozilla.org / en-US / docs / Web / JavaScript / Reference / Statements / var ）：

On Mr Lister's advice, a bit on variable hoisting. From MDN (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var):

变量声明，无论它们出现在哪里，都会在任何代码之前处理用var声明的变量的范围是它的当前执行上下文，它是封闭函数，或者对于在任何函数之外声明的变量，是全局的。

"Variable declarations, wherever they occur, are processed before any code is executed. The scope of a variable declared with var is its current execution context, which is either the enclosing function or, for variables declared outside any function, global."

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