Multiple left-hand assignment with JavaScript

Question

var var1 = 1,
    var2 = 1,
    var3 = 1;

This is equivalent to this:

var var1 = var2 = var3 = 1;

I'm fairly certain this is the order the variables are defined: var3, var2, var1, which would be equivalent to this:

var var3 = 1, var2 = var3, var1 = var2;

Is there any way to confirm this in JavaScript? Using some profiler possibly?

Answer 1

Try this:

var var1=42;
var var2;

alert(var2 = var1); //show result of assignment expression is assigned value
alert(var2); // show assignment did occur.

Note the single '=' in the first alert. This will show that the result of an assignment expression is the assigned value, and the 2nd alert will show you that assignment did occur.

It follows logically that assignment must have chained from right to left. However, since this is all atomic to the javascript (there's no threading) a particular engine may choose to actually optimize it a little differently.

Answer 2

Assignment in javascript works from right to left. var var1 = var2 = var3 = 1;.

If the value of any of these variables is 1 after this statement, then logically it must have started from the right, otherwise the value or var1 and var2 would be undefined.

Although it will not compile, you can think of it as equivalent to var (var1 = (var2 = (var3 = 1))); where the inner-most set of parenthesis is evaluated first.

Answer 3

Actually,

var var1 = 1, var2 = 1, var3 = 1;

is not equivalent to:

var var1 = var2 = var3 = 1;

The difference is in scoping:

function good() {
  var var1 = 1, var2 = 1, var3 = 1;
}

function bad() {
  var var1 = var2 = var3 = 1;
}

good();
alert(window.var2); // undefined

bad();
alert(window.var2); // 1. Aggh!

Actually this shows that assignment are right associative. The bad example is equivalent to:

var var1 = (window.var2 = (window.var3 = 1));

Answer 4

Further info can be found here:

Operator Precedence