Functional Programming

1. The Two Pillars of JavaScript, Part 2 (Functional Programming): How To Stop Micromanaging Everything
2. Wikipedia: Functional Programming

Map, Reduce & Filter

These 3 functions are very important in a functional programmers arsenal. They have long been available through libraries like Lodash, but are now commonly available on the Array.prototype.

• http://cryto.net/~joepie91/blog/2015/05/04/functional-programming-in-javascript-map-filter-reduce/

Be sure to check out the full list of Array.prototype methods (can be found at MDN), and also all collection and array methods in the LoDash documentation.

Filter

Easiest to grasp. Simply returns an new Array without any values that do not pass the predicate.

Map

Essentially maps one value to another.

Can be used to some cool stuff sith String.protoype.split and String.prototype.join, eg str.sprit(',').map(func).join().

TODO Example of above

Reduce

Often used to reduce several values into one out, but can also be used to construct new arrays or objects.

A good example of a "constructing" reduce is replacing a arr.filter().map() chain. Instead of having to iterate over and create two new arrays, we could simply

arr = arr.reduce(function(seed, value) {
    if(test(value)) {
        seed.push(manipulat(value);
    }

    return seed;
}, []);

Try To Avoid Loop

.forEach loop is simple to replace with a for loop. Even _.forEach on objects is now easily implemented with for-of loops, but these loops should generally be avoided. Methods using callbacks to iterate should be preferred because:

1. More modular code, you can reuse a previous function
2. Can handle asyncronous data. Ie data generated from events or streams.

The only reason to use good old for loops these are for performance.

Books

• O'Reilly, Functional JavaScript: Introducing Functional Programming With Underscore.js (2013)