Javascript the good parts:can't understand method array.push

Question

I'm reading Douglas Crockford's Javascript the good parts,but I can't understand the implemention of Array.push in the chapter 8 Methods,as follows:

Function.prototype.method = function(name,func){
    if(!this.prototype[name]){
        this.prototype[name] = func;
    }
};

Array.method('mypush',function(){
    this.splice.apply(this,[this.length,0].concat(Array.prototype.slice.apply(arguments)));

    return this.length;
});

var arr = [1,2,3];
arr.mypush(2,3);
console.log(arr);

I can't understand this statement:

this.splice.apply(this,[this.length,0].concat(Array.prototype.slice.apply(arguments)));

Any help would be greatly appreciated, Thanks

zerkms · Accepted Answer · 2014-11-13 03:07:28Z

5

Do that from the inside out:

Array.prototype.slice.apply(arguments) --- converts arguments array-like object into a real array
[this.length,0].concat(#1) --- concatenates the hardcoded [this.length,0] array with the array from #1
this.splice.apply(this, #2) --- applies the this.splice function to this object with arguments from #2

Finally it will look like: this.splice(this.length, 0, arg1, arg2, arg3) which means: at the index equal to this.length (see "in the very end) replace 0 elements (see - don't remove anything) with the given arguments.

References:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice

answered Nov 13, 2014 at 3:07

zerkms

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Kapol · Accepted Answer · 2016-04-13 08:30:14Z

apply takes a function and calls it on the supplied this object and parameters taken from an array.

this.splice.apply(this,[this.length,0].concat(Array.prototype.slice.apply(arguments)));

So, what we have here is:

this.splice        // a reference to the function "splice" that an Array has

this.splice.apply  // invoke this function 

this.splice.apply(this   // invoke it on "this" 

this.splice.apply(this,[     // with an array of parameters

    [this.length,0]   // the first two parameters are "the length", and a "0"

    // calling splice this way means replacing the 0 elements 
    // after the end of the array with the following new elements
    // => i.e. pushing the new elements 

    ].concat     // but we add some more parameters

    Array.prototype.slice.apply(arguments)    
        // namely the arguments passed into this function

That last step is necessary because arguments is not a real array, so it does not work with concat. slice makes a copy of it (into a real array).

Summing it all up with an example.

If you called [1,2,3].mypush(2,3) it translates into [1,2,3].splice(3, 0, 2, 3);

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