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K-Sato
K-Sato

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JavaScript Basic Array Functions

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];
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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]
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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]
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slice()

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

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

forEach()

Calls a function for each array element.

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

Check if an array contains the specified element.

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

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

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

Discussion (33)

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