Using slice() method on Array like Object

Question

I'm confused with the output.

var arrLike = {0:'Martin', 1:78, 2:67, 3:['L', 'M', 'P'], length: 4};

If I use slice() on arrLike :

var newArr = Array.prototype.slice.call(arrLike, 0);

Output:

console.log(newArr);
["Martin", 78, 67, Array[3]]

How does this happen? I can't wrap up my head around with the output.

Answer 1

Preface: Note that the Array[3] in your console output is just how the console is showing it to you. Your newArr is really:

["Martin", 78, 67, ['L', 'M', 'P']]

---

It happens because that's how slice is defined. Loosely speaking it:

1. Creates a new, empty array
2. Reads the length property of what you give it
3. Loops through from the starting index (default 0 ) to the ending index (default length - 1 ); call the loop variable k
   1. Puts any property found in the object when asking it for property k into the array at index k - start .
4. Sets the length of the returned array to k - start .
5. Returns the array

Or (again very loosely):

function slice(start, end) {
    var n, k, result;
    start = arguments.length < 1 ? 0 : +start;
    end = arguments.length < 2 ? +this.length : +end;
    result = [];
    for (var k = start, n = 0; k < end; ++k, ++n) {
        if (n in this) {
            result[n] = this[k];
        }
    }
    result.length = n;
    return result;
}

All the gory details .

Since your arrLike object has a length property and has properties with the names 0 , 1 , 2 , and 3 , you get the result you've shown.

Answer 2

Check out the spec for slice() . The slice() function will default the end parameter, which you did not specify, to this.length , and...

7. If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).

Therefore, you're only pushing the first 4 non-element properties of the arrLike object into the new array.

Try using...

var arrLike = {0:'Martin', 1:78, 2:67, 3:['L', 'M', 'P'], length: 6, 4: 'test'};