Array.prototype.slice weird behaviour

Question

Consider this piece of code, with console output at the end of each line:

function whatever() {
  console.log(arguments) // { '0': 1, '1': 2, '2': 3, '3': 4, '4': 5 }
  console.log(Array.prototype.slice.call(arguments)) // [ 1, 2, 3, 4, 5 ]
  console.log(Array.prototype.slice.call({ '0': 1, '1': 2, '2': 3, '3': 4, '4': 5 })) // []
}

whatever(1,2,3,4,5)

Why is it that the third console.log outputs an empty array?

Answer 1

Because in order for Array.prototype.slice to work, you need to pass an array-like object. And in order for an object to fit that category, it needs a length property, which your object doesn't have. Try this:

var arr = { '0': 1, '1': 2, '2': 3, '3': 4, '4': 5 };
arr.length = 5;
var res = Array.prototype.slice.call(arr);
console.log(res);

FIDDLE

Answer 2

As @basilikum described, this is because .length is required in order to use .slice() . To understand why it's required, imagine that you were writing your own version of Array.prototype.slice() after reading the MDN docs:

---

Syntax

Array.slice(begin[, end])

Parameters

begin

Zero-based index at which to begin extraction.

As a negative index, begin indicates an offset from the end of the sequence. slice(-2) extracts the second-to-last element and the last element in the sequence.

end

Zero-based index at which to end extraction. slice extracts up to but not including end .

slice(1,4) extracts the second element through the fourth element (elements indexed 1, 2, and 3).

As a negative index, end indicates an offset from the end of the sequence. slice(2,-1) extracts the third element through the second-to-last element in the sequence.

If end is omitted, slice extracts to the end of the sequence.

---

To handle all those cases and a few more not listed, your code would have to be something along these lines (this may have bugs but should be close):

Array.prototype.myslice = function( begin, end ) {
    // Use array length or 0 if missing
    var length = this.length || 0;
    // Handle missing begin
    if( begin === undefined ) begin = 0;
    // Handle negative begin, offset from array length
    if( begin < 0 ) begin = length + begin;
    // But make sure that didn't put it less than 0
    if( begin < 0 ) begin = 0;
    // Handle missing end or end too long
    if( end === undefined  ||  end > length ) end = length;
    // Handle negative end (don't have to worry about < 0)
    if( end < 0 ) end = length + end;
    // Now copy the elements and return resulting array
    var result = [];
    for( var i = begin;  i < end;  ++i )
        result.push( this[i] );
    return result;
};

That's why .slice() requires this.length —you wouldn't be able to write the function without it.

Answer 3

As much as i have knowledge

Argument is a variable of object type which we can use to get entry for every argument passed to a method

for example if we use this

whatever(a,b,c)

argument will return some thing like 0:a ,1:b ,2:c

and slice method is used to slice an array from a starting point to end point like

var myarray=["1","2","3","4"];
myarray.slice(2,3);

will return 3 and 4 as they exist on index 2 and 3

so if u want to use slice on your arguments just define it like slice(startindex,endindex);

just an edit slice.call is used to convert an array type to another array type data structure and in your case while passing arguments as it is a known type for javascript engine it considers it as an array type and simply converts it but hardcoding an array doesn't seems to work (just a thought).