Reduce function to take an undefined initial value

Question

I want to create a function (reduce) that does the following:

Where:

var collection = [1, 2, 3];

and

function iterator(         


        

Answer 1

This is how I would implement it:

alert(reduce([1,2,3], add, 3)); // 9
alert(reduce([1,2,3], add));    // 6

function add(a, b) {
    return a + b;
}

function reduce(array, iterator, initial) {
    var length = array.length;
    var index  = 0;

    if (arguments.length < 3) {
        if (length > 0) var result = array[index++]; // Note 1
        else throw new Error("Reduce of empty array with no initial value");
    } else var result = initial;

    while (index < length) result = iterator(result, array[index++]);

    return result;
}

The code is pretty self explanatory. Nevertheless, here's how it works, if the number of arguments passed are less than 3 it means that initial was not given. Hence we set result to array[0] and increment index. If array is empty then we throw an error instead. Otherwise we set the result to the initial value passed to the function. Everything else works as normal.

Note 1: The reason we don't modify initial (i.e. write initial = array[index++]) is because if we use arguments in a function and also modify the parameters of the function then the function will not be optimized in V8. Hence, it will execute slower.

Hope this helps.