Why should I use 'i in array' inside my own custom 'array.indexOf'?

Question

I have seen many times things like this:

function customArrayIndexOf(item, array){
    for (var i = 0, l = array.length; i < l; i++) {
        if (i in array && array[i] === item) return i;
    }
    return -1;
}

However, I am not sure why is the i in array needed.

I have three questions:

• What it is doing?
• Is it necessary?
• When it will fail without it?

Answer 1

Sparse arrays may cause a false positive without the check:

var customArrayIndexOf = function(item, array){
    for (var i = 0, l = array.length; i < l; i++) {
        if (i in array && array[i] === item) return i;
    }
    return -1;
},
customArrayIndexOfNoCheck = function(item, array){
    for (var i = 0, l = array.length; i < l; i++) {
        if (array[i] === item) return i;
    }
    return -1;
};

var t=[]; t[1]=1;

customArrayIndexOfNoCheck(undefined, t); // 0
customArrayIndexOf(undefined, t); // -1

(i and l should be local, i.e. declared with var)