In JavaScript, is chained assignment okay?

Question

Am not new to JS or its syntax, but sometimes, the semantics of the language has me stumped at times. At work today, a colleague mentioned this:

var a = b = []; 

is not the same as

var a = [], b = []; 

or

var a = []; var b = []; 

since the first version actually assigns the reference to an empty array to a and b. I couldn't quite accept this as true, but I'm not sure. What do you all think?

Answer 1

Yes, they're not the same. var a = b = [] is equivalent to

var a; b = []; a = b; 

Not only do both a and b get assigned the same value (a reference to the same empty array), b is not declared at all. In strict mode in ECMAScript 5 and later, this will throw a ReferenceError; otherwise, unless there is already a variable b in scope, b is silently created as a property of the global object and acts similarly to a global variable, wherever the code is, even inside a function. Which is not good.

You can see this quite easily:

(function() {     var a = b = []; })();  console.log(b); // Shows []