Multiple variables assigned to one variable in Javascript?

Question

I'd like to know what kind of syntax the line below is called.

var that = {}, first, last;

Note: I found a post on this site about this question, but they said [ ] has to be added around the variables on the right handside to make it an array. But the code below does work.

Code:

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;
};

thefourtheye · Accepted Answer · 2014-12-29 14:28:37Z

var that = {}, first, last;

is similar to

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

We are initializing that with an empty object, whereas first and last are uninitialized. So they will have the default value undefined.

JavaScript assigns values to the variables declared in a single statement from left to right. So, the following

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

will print

{} undefined {}

where as

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

would print

undefined undefined 1

Because, by the time that is assigned last, the value of last is not defined yet. So, it will be undefined. That is why that is undefined.

Paul Roub · Accepted Answer · 2014-12-29 14:23:54Z

1

It's just a shorthand way to create multiple variables. It might be more clear if written as:

var that = {}, 
    first, 
    last;

And is equivalent to:

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

answered Dec 29, 2014 at 14:23

Paul Roub

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