CODEWITHSUNDEEP
javascriptarraysprototypeslice
Mr Mikkél
Mr Mikkél

Reputation: 2643

JavaScript: why use prototype to call a function instead of just calling the function?

Why would you slice an array like this:

Array.prototype.slice.call(arr, 3);

Instead of simply:

arr.slice(3);

?

What are the benefits of using the prototype and the call?

Thanks!

Upvotes: 3

Views: 304

Answers (1)

Pointy
Pointy

Reputation: 413757

The key benefits are realized when "arr" is not an array, but something like an array; specifically, something with a "length" property and numerically-keyed properties. Good examples are the arguments object and NodeList objects from a DOM. Those things won't have a "slice" method, but they do have numerically-keyed properties and a "length" property.

The trick works because the "slice" method is pretty forgiving.

If you see it being used with something that's definitely an array already, then you're looking at code written by a confused person :)

Oh, and note also that a short-cut is:

var foo = [].slice.call(arguments, 0);

You don't have to go to the prototype directly if you don't want to; you can get at it from an array instance just as easily. (Yes, it costs a dummy allocation, but in this day and age I wouldn't be surprised if VMs optimized that away.)

Upvotes: 6

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