JS Functional Programming.md

Functional Programming

Fundamentals

Functional programming is the name given to the concept of calling a generic iterator function with each member in a collection, and producing a result consistent with a standard programming paradigm.

At least once you grok it, functional programming is super-easy in JavaScript, since JS will treat functions like any other variable, and so they can be easily passed as a parameter to another function.

Patterns

Pretty much everything in functional programming can be distilled down to one of three patterns, which can be recognized by their purpose and by their output.

• map: Takes in an array. Returns an array with each item from the original array transformed somehow.

• filter: Takes in an array. Returns an array of equal or shorter length, containing a specific subset of items.

• reduce: Takes in an array. Returns a single value that synthesizes all values in the array.

Let's discuss each in detail.

map

Mapping is useful for transforming the members of an array.

Map iterator functions have the parameter signature of function(value, index).

var doubler = function(value, index) {
    return value * 2;
}
var squarer = function(value, index) {
    return value * value;
}

var array = [1, 2, 3, 4];

var doubledArray = array.map(doubler);
var squaredArray = array.map(squarer);

console.log(doubledArray); // [ 2, 4, 6, 8 ]
console.log(squaredArray); // [ 1, 4, 9, 16 ]

filter

Filtering is useful for, well, filtering the members of an array into a specific subset.

Like map, filter iterator functions have the parameter signature of function(value, index).

var odds = function(value, index) {
    return (value % 2 == 1);
}
var greaterThan2 = function(value, index) {
    return (value > 2);
}

var array = [1, 2, 3, 4];

var oddArray = array.filter(odds);
var greaterThanArray = array.filter(greaterThan2);

console.log(oddArray); // [ 1, 3 ]
console.log(greaterThanArray); // [ 3, 4 ]

reduce

Reducing is useful for distilling information from the members of an array into a single value.

Unlike map and filter, reduce functions require another value when being called -- the seed value that the memory starts at.

Reduce iterator functions have the parameter signature of function(memory, value, index).

var joinAsString = function(memory, value, index) {
    return memory + ", " + value;
}
var sum = function(memory, value, index) {
    return memory + value;
}

var array = [1, 2, 3, 4];

var stringOfArray = array.reduce(joinAsString, "");
var sumOfArray = array.reduce(sum, 0);

console.log(stringOfArray); // "1, 2, 3, 4, "
console.log(sumOfArray); // 10