array.unshift invoked using apply() causes “Maximum call stack size exceeded”

Question

This code:

var arr = [];
arr.unshift.apply(arr, new Array(200000));

causes: Uncaught RangeError: Maximum call stack size exceeded (see http://jsfiddle.net/BnLxf/1/ )

While this code works fine:

var arr = [];
arr.unshift(new Array(200000));

Any idea why this happens?

Answer 1

Your first example is equivalent to this:

 arr.unshift(undefined, undefined, undefined, /* undefined 199,997 more times */);

Your second example just passes unshift one argument, which is an array with no entries and a length of 200000 .

This is because the purpose of Function#apply is to call a function with a given this value of arguments which are then then passed onto the target function as discrete (individual) arguments.

It's not entirely surprising that a JavaScript engine chokes when asked to call a function with 200,000 discrete arguments. (I also wouldn't have been surprised if it worked.)

If you want to pass the array as just a single argument, but you want to use something apply -like, use call :

arr.unshift.call(arr, new Array(200000));

call passes on the arguments you give it exactly as you give it to them (except the first one, which is what to use as this during the call).

Answer 2

The Function#apply method expects an array as 2nd parameter. You should embrace new Array(200000) with brackets:

var arr = [];
arr.unshift.apply(arr, [new Array(200000)]);

As Chris said, you can also call method .call instead of .apply :

var arr = [];
arr.unshift.call(arr, new Array(200000));