How to find the index of a missing value in an array?

Question

I have an array like

a=[1,2,3,,4,5];

So now I want to find the missing value index, i.e. 3, using indexOf.

Answer 1

Check if the value is undefined like the following code:

for ( var i = 0; i < a.length; i++ ) {
    if ( typeof a[i] === "undefined" ) {
        // do stuff here or break the loop
    }
}

Update You can do this too:

Array.prototype.indexOfUndefined =  function() {
    for ( var i = 0; i < this.length; i++ ) {
        if ( typeof this[i] === "undefined" ) {
            return i;
        }
    }
}

You need to return i because i is the current index, it will search for the first undefined value.

Demo: http://jsfiddle.net/vdyypq6o/5/

Answer 2

Unfortunately, ES5 Array methods are required* to skip such array holes**, so no indexOf() or forEach() will help. ECMAScript 2015 has two new methods called find() and findIndex() that could help you, but they are not widely supported yet, so I assume it's not a good answer for this question.

What's left is a good old iteration over indexes:

function findHole(a) {
    for (var i = 0; i < a.length; i++) {
        // check for only `a[i] === undefined` could be faster,
        // but is not enough as it will cause false positives
        // when array actually contains `undefined` value
        // for example, for `a = [1, undefined, , 2]`,
        // correct implementation should return `2`, not `1`
        if (a[i] === undefined && !a.hasOwnProperty(i)) {
           console.log("Found hole at index " + i);
           return i;
        }
    }
    return -1;
}

* — see step 9 in http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.14 for indexOf() and step 8 http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.19 for map() algorithms that require skipping holes

** — a hole is created in array when there's no value defined for some index, so technically it's not correct to say that a[3] has null or undefined value, because there just isn't any value. But in JavaScript, when we try to get not defined property of some object, what we get is undefined, that why a[3] === undefined is true.