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']
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
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