Problem with your code

Your existing code fails because the check for recursing is backward. You want to recurse if the length is non-zero. So it should be

  if (array[count].length) getMultiLength(array[count]);

  else sum++;

As your code stands, getMultiLength will be called even if array[count] is not an array (because if array[count] is not an array, length will be undefined). So it will keep recursing forever. This would be pretty easy to figure out by just stepping through your code in the debugger.

By the way, you don't need this.valueOf(). That is the same as this in this case.

Tweaking your code

But actually, you could streamline your code by eliminating the unnecessary inner function, and using the return value of the recursive calls:

Array.prototype.getLength = function() {

  let sum = 0;

  for (let count = 0; count < this.length; count ++) {

    sum += this[count].length ? this[count].getLength() : 1;

  }

  return sum;

};

Some people might prefer to write this using reduce:

Array.prototype.getLength = function() {

  return this.reduce((sum, elt) => 

    sum + (elt.length ? elt.getLength() : 1), 0);

};

Another solution using flattening

An alternative solution is to flatten the array, then find the length of the flattened array. Here we use a generator to create a flattener which is real easy to read and understand (ES6 feature):

function *flatten(array) {

  for (elt of array) 

    if (Array.isArray(elt)) yield *flatten(elt);

    else yield elt;

}



var testArray = [1,2,[3,4,[5,6],7],8,9,[10,11],12];



console.log(Array.from(flatten(testArray)).length);

Alternative implementation of flatten

Or, use your own favorite implementation of flatten, such as this recursive version:

function flatten(value) {

  return Array.isArray(value) ? [].concat(...value.map(flatten)) ? value;

}

or in ES5

function flatten(value) {

  return Object.prototype.toString.call(value) === '[object Array]' ?

    [].concat.apply([], value.map(flatten)) :

    value;

}

Putting it on the Array prototype

If you insist on putting this on the prototype, then

Object.defineProperty(Array.prototype, 'getLength', {

  value() { return flatten(this).length; }

});

Use defineProperty to make this property non-enumerable, non-configurable etc.