Array.reduce

ArrayReduce.js

/*
For each item in the array,
pass the item into a function with the last value returned from the function.
If it's the first item, pass the second parameter (starting value) to reduce in.
If no starting value is passed, use the first item in the array, and skip to the second.
If no staring value is passed, and the array is empty, throw an error.
*/

function reduce(yourFunction, startingValue) {
    var items = this;

    var hasStartingValue = arguments.length === 2;

    var currentResult = startingValue;

    if (hasStartingValue) {
        currentResult = startingValue;
    } else {
        if(!items.length) {
            throw 'Reduce of empty array with no initial value';
        }

        currentResult = items[0];
    }

    for (var i = hasStartingValue ? 0 : 1; i < items.length; i++) {
        currentResult = yourFunction(currentResult, items[i]);
    }

    return currentResult;
}

Array.prototype.reduce = reduce;

var output = [1, 2, 3].reduce((lastResult, nextItem) => {
    var newCurrentResult = lastResult + nextItem;

    return newCurrentResult;
});

console.log(output); // -> 1 + 2 + 3 == 6