Getting Started With Functional Programming

Doubling an Array of Numbers

function doubleValues(arr) {
    var newArr = []
    for(var i = 0; i < arr.length; i++) {
        newArr.push(arr[i] * 2)
    }
    return newArr
}
/*
calling doubleValues([1, 2, 3, 4])
gets us [ 2, 4, 6, 8 ]
*/

Filtering Out Odd Numbers

function evenValues(arr) {
    var newArr = []
    for(var i = 0; i < arr.length; i++) {
        if (arr[i] % 2 === 0) {
            newArr.push(arr[i])
        }
    }
    return newArr
}
/*
calling evenValues([1, 2, 3, 4])
gets us [ 2, 4 ]
*/

A Sum of a List of Numbers

function sumValues(arr) {
    var sum = 0
    for(var i = 0; i < arr.length; i++) {
        sum += arr[i];
    }
    return sum
}

/*
calling sumValues([1, 2, 3, 4])
gets us 10
*/

First Class Functions

function someFunction() {
    return 'this is from some function'
}


/*
calling someFunction()
gets us 'this is from some function'
*/

First Class Functions

const theSameFunction = someFunction


/*
calling theSameFunction()
gets us 'this is from some function'
*/

First Class Functions

function callFunction(f) {
    return f()
}


/*
calling callFunction(someFunction)
gets us 'this is from some function'
*/

Doubling an Array of Numbers

function doubleValues(arr) {
    return arr.map(function (x) {
        return 2 * x
    })
}


/*
calling doubleValues([1, 2, 3, 4])
gets us [ 2, 4, 6, 8 ]
*/

Filtering Out Odd Numbers

function evenValues(arr) {
    return arr.filter(function (x) {
        return x % 2 === 0 }
    )
}


/*
calling evenValues([1, 2, 3, 4])
gets us [ 2, 4 ]
*/

A Sum of a List of Numbers

function sumValues(arr) {
    return arr.reduce(function (sum, x) {
        return sum + x
    })
}


/*
calling sumValues([1, 2, 3, 4])
gets us 10
*/

Arrow Functions

const add  = (x, y) => x + y

const double_num = x => 2 * x

//by rights should be a one-liner
//but just to show you another way...
const triple_add = (x, y, z) => {
    return 3 * (x + y + z)
}
/*
add(1, 2) gets us 3

double_num(5) gets us 10

triple_add(1, 2, 3) gets us 18
*/