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javascriptprototype
Wild Widow
Wild Widow

Reputation: 2549

javascript concat using call method

I'm trying to concat two arrays in javascript using concat() method. I'm wondering what is the difference between these two cases

var a = [1, 2, 3];
console.log(Array.prototype.concat.call(a, [4, 5, 6]));
// Result: [1, 2, 3, 4, 5, 6]

console.log(Array.prototype.concat(a, [4, 5, 6]));
// Result: [1, 2, 3, 4, 5, 6]

I thought by using call method, as we are passing in this object as the first argument, it would change the array a. But it didn't.

Upvotes: 0

Views: 765

Answers (2)

Bergi
Bergi

Reputation: 664599

Array.prototype.concat.call(a, [4, 5, 6])

is equivalent to a.concat([4, 5, 6]) (assuming a.concat === Array.prototype.concat, which is the case in your example where a instanceof Array).

Array.prototype.concat(a, [4, 5, 6])

is equivalent to Array.prototype.concat.call(Array.prototype, a, [4, 5, 6]).

Given that Array.prototype is an empty array, both yield the same result. [].concat(a, [4, 5, 6]) or Array.prototype.concat.call([], a, [4, 5, 6]) would as well.

Upvotes: 2

slessans
slessans

Reputation: 687

Array.prototype.concat returns a new array. So Array.prototype.concat(a, [4, 5, 6]) and Array.prototype.concat.call(a, [4, 5, 6]) actually do the same thing. If you want to change the original array, you should, use Array.prototype.push.apply(a, [4, 5, 6])

Upvotes: 2

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