Example of array methods: forEach, map, filter, some, every, indexOf, and include. (ES2015 syntax)

arrayMethods.js

/*
*     forEach
*/
// forEach is very simlar to for-looping arr.length
// array.forEach(callback, context)
var arr = ['apple', 'orange', 'watermelon', 10, 20, 30]

arr.forEach((value, index) => console.log(`Element's ${index} type is ${typeof value}`))
// Prints:
// Element 0 type is string
// Element 1 type is string
// Element 2 type is string
// Element 3 type is number
// Element 4 type is number
// Element 5 type is number

/*
*     map
*/
// array.map(callback, context)
// returns an array
// will return a new array of elements from 'array' after applying the callback function on each element
// alyways returns same size of array
var arr2 = [
  { fruit:'apple', color:'Red' },
  { fruit:'orange', color:'Blue' },
  { fruit:'watermelon', color:'Green' }
]

var arr2map = arr2.map(obj => obj.color)
console.log(arr2map)
// Prints:
// [ 'Red', 'Blue', 'Green' ]


/*
*     filter
*/
// arr.filter(callback, context)
// will return a new array with item that only satisfy a specific condition we provide in the callback function
var arr = ['apple', 'orange', 'watermelon', 10, 20, 30]
var numbersOnly = arr.filter(num => typeof num === 'number')
console.log(numbersOnly)
// Prints:
// [ 10, 20, 30 ]


/*
*     some
*/
// arr.some(callback, context)
// returns boolean
// will apply a function on each element in the array and return true once it finds one, or false if the list has been exhausted

var arr3 = [10, 20, 30]
var checkFor = num => arr3.some( element => element === num )
console.log(checkFor(20)); // true
console.log(checkFor(40)); // false


/*
*     every
*/
// arr.every(callback, context)
// returns a boolean
// will stop and return false once it finds an elements that doesn't match the condition in the callback function
var arr4 = [10, 20, 30]
var arr4every = num => arr4.every( element => element > num )
console.log(arr4every(9)); // true
console.log(arr4every(15)); // false
console.log(arr4every(40)); // false

/*
*     indexOf
*/
// arr.indexOf(value, fromIndex)
// returns a 'number' of which that value is at from the beggining of the array, or (optionally) from 'fromIndex'
// or returns '-1' if not exist
var arr5 = ['Green', 'Red', 'Blue', 'G']
console.log(arr5.indexOf('blue')) // -1
console.log(arr5.indexOf('Blue')) // 2
console.log(arr5.indexOf('Green')) // 0
console.log(arr5[2]) // Blue


/*
* includes
*/

const a = [1, 2, 3];
a.includes(2); // true 
a.includes(4); // false