Is setting multiple variables in 1 line valid in javascript? (var x=y='value';)

Question

This is valid in php:

$x=$y=\'value\';

This will in esscence set both $x and $y to \'value\'.

Is this valid in javascript?

Answer 1

I'm pretty sure that the most readable form is the one that has not been brought up yet:

let x = 0, y = 0

Answer 2

To prevent the y from becoming a global variable, use the following:

var x, y = x = 'value';

Apparently the declarations are simply evaluated from left to right.

Answer 3

When vars have different values you can easily write:

var x = 0,
    y = 1,
    ...
    z = 2;

Answer 4

Yes, that is valid in Javascript.

However, after googling the problem, this Stack Overflow question explains some caveats, so use with caution.

Answer 5

It only works if the var y as been previously defined, otherwise ywill be global.

In such case, you better do:

var x, y;
x = y = 'value';

Assignments Chaining

Another antipattern that creates implied globals is to chain assignments as part of a var declaration. In the following snippet, a is local but b becomes global, which is probably not what you meant to do:

// antipattern, do not use
function foo() {
   var a = b = 0;
   // ...
}

If you’re wondering why that happens, it’s because of the right-to-left evaluation. First, the expression b = 0 is evaluated and in this case b is not declared. The return value of this expression is 0, and it’s assigned to the new local variable declared with var a. In other words, it’s as if you’ve typed:

var a = (b = 0);

If you’ve already declared the variables, chaining assignments is fine and doesn’t create unexpected globals. Example:

function foo() {
   var a, b;
   // ...
   a = b = 0; // both local
}

“JavaScript Patterns, by Stoyan Stefanov (O’Reilly). Copyright 2010 Yahoo!, Inc., 9780596806750.”