Introduction

Here are some of the most fundamental JS array functions.

Assume the code below is declared at the top level in the following examples.

var arr = [1, 2, 3, 4, 5];

Add or Remove items

pop()/push()

pop(): Removes the last element of an array, and returns that element.
push(): Adds new elements to the end of an array, and returns the new length.

arr.push(6);
console.log(arr); //=> [1, 2, 3, 4, 5, 6]

arr.pop(6);
console.log(arr); //=>  [1, 2, 3, 4, 5]

shift()/unshift()

shift(): Removes the first element of an array, and returns that element.
unshift(): Adds new elements to the beginning of an array, and returns the new length.

arr.unshift(0);
console.log(arr); //=> [0, 1, 2, 3, 4, 5]

arr.shift();
console.log(arr); //=>  [1, 2, 3, 4, 5]

slice()

Selects a part of an array, and returns the new array.

let a4 = arr.slice(0, 3);
console.log(a4); //=>  [1, 2, 3]

splice()

Adds/Removes elements from an array.

var months = ['Jan', 'March', 'April', 'June'];
months.splice(1, 0, 'Feb');
// inserts at 1st index position
console.log(months);
// expected output: Array ['Jan', 'Feb', 'March', 'April', 'June']

months.splice(4, 1, 'May');
// replaces 1 element at 4th index
console.log(months);
// expected output: Array ['Jan', 'Feb', 'March', 'April', 'May']

• JavaScript - Array splice() Method - Tutorialspoint

concat()

Joins two or more arrays, and returns a copy of the joined arrays.

let a1 = arr.concat([6, 7]);
console.log(a1); //=> [1, 2, 3, 4, 5, 6, 7]

Iterate items

forEach()

Calls a function for each array element.

function repeat(ele) {
  console.log(ele);
}
arr.forEach(repeat); //=> 1 2 3 4 5

Search in array

indexOf()

Looks for item starting from index from, and returns the index where it was found, otherwise -1.

console.log(arr.indexOf(0)) //=> 1
console.log(arr.indexOf(10)) //=> -1

find()/findIndex()

Returns the value of the first element in an array that pass a test.

console.log(arr.find(num => num > 2)); //=> 3
console.log(arr.findIndex(num => num > 2)); //=> 2

includes()

Check if an array contains the specified element.

console.log(arr.includes(2)); //=> true

filter()

Creates a new array with every element in an array that pass a test.

let a2 = arr.filter(num => num > 3);
console.log(a2); //=> [4, 5]

Transform array

map()

Creates a new array with the result of calling a function for each array element.

a3 = arr.map(ele => ele - 1);
console.log(a3); //=> [0, 1, 2, 3, 4]

sort()

Sorts the elements of an array.

let array = [4, 2, 3, 4, 5, 6, 7];
console.log(array.sort()); //=> [2, 3, 4, 4, 5, 6, 7]

reverse()

The method arr.reverse reverses the order of elements in arr.

console.log(arr.reverse()) //=> [5, 4, 3, 2, 1];

reduce()

The reduce() method reduces the array to a single value.

The reduce() method executes a provided function for each value of the array (from left-to-right).

Here is the basic syntax.

let value = arr.reduce(function(previousValue, item, index, array) {
  // ...
}, initial);

• item – is the current array item.
• index – is its position.
• array – is the array.
• previousValue – is the result of the previous function call, initial for the first call.

let result = arr.reduce((sum, current) => sum + current, 0);

console.log(result); //=> 15

references

• w3schools.com
• javascript.info