CODEWITHSUNDEEP
javascriptarraysfunctionarguments
Viktor Stolbin
Viktor Stolbin

Reputation: 2939

How JS 'arguments' object gets converted to array

Looking for more details on what exactly is happening when following is executed:

    function useArray(){
        var args = [].slice.call(arguments)
        console.log(args)
    }

How come slice being a function with 3 parameters (source array, start and end positions to copy) threats arguments correctly? Method call needs correct value of first argument as this but it seems arguments gets turned into Array object?

And why this doesn't work:

var args = [].slice(arguments)

Upvotes: 1

Views: 64

Answers (1)

Paul Roub
Paul Roub

Reputation: 36448

We're using [].slice to get at the Array::slice method. arguments doesn't have a slice() of its own, but it is array-like.

Although it's a member of Array, slice() will work well enough on array-like objects -- things that have .length and can be indexed with [0..n].

slice() with no parameters returns a copy of the entire array. Its arguments are both optional.

So it's as if arguments had a slice() method, and we called arguments.slice() to get a copy of it (as an array).

[].slice(arguments) is just calling Array::slice() on an empty array, with arguments as the begin parameter, which makes no sense since begin (if present) should be a number.

Upvotes: 1

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