javascript - 在 Javascript 中将多个变量分配给一个变量？

Question

我想知道下面这行调用的是哪种语法。

var that = {}, first, last;

注意:我在这个网站上找到了一篇关于这个问题的帖子，但他们说必须在右侧的变量周围添加 [ ] 才能使其成为数组。但是下面的代码确实有效。

代码:

var LinkedList = function(e){

  var that = {}, first, last;

  that.push = function(value){
    var node = new Node(value);
    if(first == null){
      first = last = node;
    }else{
      last.next = node;
      last = node;
    }
  };

  that.pop = function(){
    var value = first;
    first = first.next;
    return value;
  };

  that.remove = function(index) {
    var i = 0;
    var current = first, previous;
    if(index === 0){
      //handle special case - first node
      first = current.next;
    }else{
      while(i++ < index){
        //set previous to first node
        previous = current;
        //set current to the next one
        current = current.next
      }
      //skip to the next node
      previous.next = current.next;
    }
    return current.value;
  };

  var Node = function(value){
    this.value = value;
    var next = {};
  };

  return that;
}; 

Answer 1

var that = {}, first, last;

类似于

var that = {};
var first;
var last;

我们正在用空对象初始化 that，而 first 和 last 未初始化。因此它们将具有默认值 undefined。

JavaScript 从左到右为单个语句中声明的变量赋值。所以，下面

var that = {}, first, last = that;
console.log(that, first, last);

将打印

{} undefined {}

在哪里

var that = last, first, last = 1;
console.log(that, first, last);

会打印

undefined undefined 1

因为，当 that 被赋值为 last 时， last 的值还没有定义。因此，它将是 undefined。这就是为什么 that 是 undefined 的原因。