[].splice.apply or [].splice.push etc

Question

over the last couple days I have been studying some array methods and have seen stuff like [].splice.apply and [].push.apply with some parameters.

Now I think i understand splice for example

var tuna = ['bob', 'john' , 'papa' , 'greg'];
var newArr = []
tuna.splice(0,2,"hello");
console.log(tuna);

will output hello,papa,greg, since it deletes 2 terms in the 0th indice.

I also know how apply works in the following context

var person = {
    firstname: 'John',
    lastname: 'Doe',
    getFullName: function() {

        var fullname = this.firstname + ' ' + this.lastname;
        return fullname;

    }
}

var logName = function(lang1, lang2) {

    console.log('Logged: ' + this.getFullName());
    console.log('Arguments: ' + lang1 + ' ' + lang2);
    console.log('-----------');

}
var logPersonName = logName.bind(person);
logPersonName('en');
logName.call(person, 'en', 'es');
logName.apply(person, ['en', 'es']);

So what happens when we combine these two methods, or push with apply ? A simple example would really help.

`[].splice.push` does not make sense. `[].splice.apply` does, because "apply" is the name of a function on the Function prototype. Now, `[].push.apply` makes sense, for a similar reason. — Pointy, Jun 29 '16 at 15:44
Yes, sorry for some reason I typed that without thinking. Thanks for the correction — Muntasir Alam, Jun 29 '16 at 15:47

score 1 · Answer 1 · answered Jun 29 '16 at 15:46

1

What you've likely seen is:

var foo = Array.prototype.slice.apply(arguments)

This takes the slice method of Array, i.e. the canonical implementation of slice, and uses it on an array-like object, and returns an array. This is a shorthand to turn an array-like object into an actual array. It is purely used because it is the shortest and most convenient way to achieve this goal. slice happens to work on any array-like object and return an actual array, and when not using any of its additional parameters it doesn't do anything else with the array; that's why it's convenient to use for this purpose. It's really abusing the slice method for a purpose it wasn't designed for, but since it works everywhere... well, here we are using it.

Example:

var o = {};
o[0] = 'foo';
o[1] = 'bar';
o.length = 2;

This is an array-like object, but it doesn't have any actual array methods. I can't do o.push('baz'), because o is not an actual array. To turn it into an actual array, I'd use the above method, and can then treat it like an actual array.

answered Jun 29 '16 at 15:46

deceze

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Thanks,do you have any examples of using .splice.apply? – Muntasir Alam Jun 29 '16 at 15:50
That's not a typical use I'm aware of, but I'm sure there are situations where it's handy. Come up with your own made-up scenarios. – deceze Jun 29 '16 at 15:51
By the way is the way you are explaining "apply" here, different from the example I made? I've only learned to use it in that context – Muntasir Alam Jun 29 '16 at 15:51
It's not fundamentally different, no. The key part to understand is that functions and objects are very loosely bound together. Even if a function is defined on an object (usually called a "method"), you can take that function and apply it to anything else you want, it doesn't have to be that original object. That is all simply decided at *call time*, not definition time. `apply` is one way to do this, to use an existing function, regardless of where it comes from, and, well, *apply* it to something else. – deceze Jun 29 '16 at 15:55

[].splice.apply or [].splice.push etc

1 Answers1