Question

我只是尝试使用以下代码检查范围变化，但结果不是以预期的方式。任何人都可以解释这背后究竟发生了什么......？

var a = 10;

+ function () {
  a = 5;
  var a;
  alert("inline  " + a); //Expected undefined but it displayed as 5
}();

alert(a); //Expected 5 but it alerted 10

DEMO

Answer 1

var hoisting

因为变量声明（和一般的声明）是在执行任何代码之前处理，在任何地方声明变量在代码中相当于在顶部声明它。这也意味着变量在声明
之前似乎可以使用

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var

编译器将您的代码理解为

var a;
a = 5;

Answer 2

函数中var声明的位置并不重要。这些完全相同：

function () {
    var a;
    a = 5;
}

function () {
    a = 5;
    var a;
}

Answer 3

使用＆＃34; var a＆＃34;你要求变量的全局值，使用＆＃34; window.a＆＃34;显示它：

<script>
var a = 10;

+ function () {
    a = 5;
    var a;
    alert("inline  " + window.a); //Expected undefined but it displayed as 5
}();

</script>

Answer 4

var a = 10; // Global scope

+ function () {
    a = 5; // You are not using Var here .. so it is also a global scope
    var a; // Now you are mentioning your a is Var so scope is changed from global to this function
    alert("inline  " + a); // Printing 5 because you local scope a has value of 5.
}();

alert(a); // Printing 10 because of global scope a has value 10

警报未按预期显示。

DEMO

4 个答案: