javascript - 为什么此代码的输出是242，而不是243

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Javascript function scoping and hoisting
（16个答案）
7个月前关闭。

var x = 2;

function fun() {
  x = 3;
  var x = 4;
  document.write(x);
}

document.write(x);

fun()

document.write(x);

有人能帮我理解控制的流程吗。为什么输出242看起来应该是243。所有的帮助将不胜感激。

最佳答案

当您修改x=3时，实际上并没有改变全局变量x，而是改变函数块中声明的变量（因为var变量具有函数作用域）。当声明被提升到顶部，然后对var x进行修改时

<script>
      var x=2;
      function fun(){
          //var x; hoisted to the top;
          console.log("x is hoisted here and uninitialized value will be", x)
	  x=3; //initialized, actually referring to the variable declared in the function scope
	  var x = 4; //declared but hoisted at the top
	  document.write(x);
	}
	document.write(x);
	fun()
	document.write(x);
</script>

要在全局范围内真正更改变量，请使用x = 3来引用它：

<script>
    	var x=2;
    	function fun(){
    		window.x=3; //modifying the global variable 'x`
    		var x = 4;
    		document.write(x);
    	}
    	document.write(x);
    	fun()
    	document.write(x);
</script>