JavaScript forEach implementation

Question

I found a code snippet for a forEach function on a tutorial website, and everything makes good sense to me except for the line that checks if i is in the array:

    if (i in this) {       

Why bother if we already have a for loop that has a stop condition?

if (!Array.prototype.forEach) {
Array.prototype.forEach = function(fun /*, thisp*/) {
    var len = this.length >>> 0;
    if (typeof fun != "function") {
        throw new TypeError();
    }

    var thisp = arguments[1];
    for (var i = 0; i < len; i++) {
        if (i in this) {
            fun.call(thisp, this[i], i, this);
        }
    }
};
}

Answer 1

Two reasons:

1. Mutation by the callback

Calling fun might change the array, since fun is entirely user-defined. So you need to check again.

Example:

array.forEach(function (el, i) { delete array[i + 1]; });

2. Sparse arrays

The other issue is that there can be sparse arrays: e.g.

3 in ["a", "b", "c", , "e", "f"] === false
// even though
3 in ["a", "b", "c", undefined, "e", "f"] === true

In those cases you don't want to call fun for that index/element, since there is nothing at that index.

["a", "b", "c", , "e", "f"].forEach(function (el, i) {
    console.log(el + " at " + i);
});
// => "a at 0" "b at 1" "c at 2" "e at 4" "f at 5"

Answer 2

Because an array can have holes, and therefore you can iterate over the length and not all values will exist.

x = new Array()
[]

x[0] = "zero"
"zero"

x[5] = "five"
"five"

x
["zero", undefined × 4, "five"]

3 in x
false

x.length
6

for (var i = 0; i < x.length; i++) { console.log(i, i in x, x[i])}
0 true "zero"
1 false undefined
2 false undefined
3 false undefined
4 false undefined
5 true "five"