JavaScript Array Iterations

Array Iteration Methods

Array iteration methods operate on every array item.

Complete JavaScript Array Reference ## Looping Array Values JavaScript provides 3 simple statements for looping over each array items:

• for ... of (loops over entries)
• for ... in (loops over indexes)
• forEach (myFunction)

The for...of Loop (Recommended)

The for...of statements hands you the array values.

Example

// Create an Array

const cars = ['BMW', 'Volvo', 'Mini'];

// Iterate over the Array values

let text = "";

for (let x of cars) {

  text += x + ",";

}

The for...in Loop (Not Recommended)

The for...in statements hands you the array indexes.

Example

// Create an Array

const cars = ['BMW', 'Volvo', 'Mini'];

// Iterate over the Array values

let text = "";

for (let x in cars) {

  text += x + ",";

}

To make it return the elements you must use [x].

Example

// Create an Array

const cars = ['BMW', 'Volvo', 'Mini'];

// Iterate over the Array values

let text = "";

for (let x in cars) {

  text += cars[x] + "
";
}

JavaScript Array forEach()

The forEach() method calls a function (a callback function) for each array element.

Example

const numbers = [45, 4, 9, 16, 25];

let txt = "";

numbers.forEach(myFunction);

function myFunction(value, index, array) {

txt += value + "**";

}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

The example above uses only the value parameter. The example can be rewritten to:

Example

const numbers = [45, 4, 9, 16, 25];

let txt = "";

numbers.forEach(myFunction);

function myFunction(value) {

txt += value + " ";

}

JavaScript Array map()

The map() method creates a new array by performing a function on each array element.

The map() method does not execute the function for array elements without values.

The map() method does not change the original array.

This example multiplies each array value by 2:

Example

const numbers1 = [45, 4, 9, 16, 25];

const numbers2 = numbers1.map(myFunction);

function myFunction(value, index, array) {

    return value * 2;

}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

When a callback function uses only the value parameter, the index and array parameters can be omitted:

Example

const numbers1 = [45, 4, 9, 16, 25];

const numbers2 = numbers1.map(myFunction);

function myFunction(value) {

    return value * 2;

}

JavaScript Array flatMap()

ES2019 added the Array flatMap() method to JavaScript.

The flatMap() method first maps all elements of an array and then creates a new array by flattening the array.

Example

const myArr = [1, 2, 3, 4, 5, 6];

const newArr = myArr.flatMap(x => [x, x * 10]);

JavaScript Array filter()

The filter() method creates a new array with array elements that pass a test.

This example creates a new array from elements with a value larger than 18:

Example

const numbers = [45, 4, 9, 16, 25];

const over18 = numbers.filter(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

In the example above, the callback function does not use the index and array parameters, so they can be omitted:

Example

const numbers = [45, 4, 9, 16, 25];

const over18 =
  numbers.filter(myFunction);

function myFunction(value) {
  return value > 18;
}

JavaScript Array reduce()

The reduce() method runs a function on each array element to produce a single value.

The reduce() method works from left-to-right in the array. See also reduceRight().

This example finds the sum of all numbers in an array:

Example

const numbers = [45, 4, 9, 16, 25];

let sum = numbers.reduce(myFunction);

function myFunction(total, value, index, array) {

  return total + value;
}

Note that the function takes 4 arguments:

• The total (the initial value / previously returned value)
• The item value
• The item index
• The array itself

Since the example above does not use the index and array parameters, it can be rewritten to:

Example

const numbers = [45, 4, 9, 16, 25];

let sum = numbers.reduce(myFunction);

function myFunction(total, value) {

  return total + value;
}

The reduce() method can accept an initial value:

Example

const numbers = [45, 4, 9, 16, 25];

let sum = numbers.reduce(myFunction,
  100);

function myFunction(total, value) {
  return total + value;
}

JavaScript Array reduceRight()

The reduceRight() method runs a function on each array element to produce a single value.

The reduceRight() works from right-to-left in the array. See also reduce().

This example finds the sum of all numbers in an array:

Example

const numbers = [45, 4, 9, 16, 25];

let sum = numbers.reduceRight(myFunction);

function myFunction(total, value, index, array) {

  return total + value;
}

Note that the function takes 4 arguments:

• The total (the initial value / previously returned value)
• The item value
• The item index
• The array itself

The example above does not use the index and array parameters. It can be rewritten to:

Example

const numbers = [45, 4, 9, 16, 25];

let sum = numbers.reduceRight(myFunction);

function myFunction(total, value) {
  return total + value;
}

JavaScript Array every()

The every() method checks if all array values pass a test.

This example checks if all array values are larger than 18:

Example

const numbers = [45, 4, 9, 16, 25];

let allOver18 =
  numbers.every(myFunction);

function myFunction(value, index, array) {

    return
  value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

When a callback function uses the first parameter only (value), the other parameters can be omitted:

Example

const numbers = [45, 4, 9, 16, 25];

let allOver18 =
  numbers.every(myFunction);

function myFunction(value) {
  return
  value > 18;
}

JavaScript Array some()

The some() method checks if some array values pass a test.

This example checks if some array values are larger than 18:

Example

const numbers = [45, 4, 9, 16, 25];

let someOver18 = numbers.some(myFunction);

function myFunction(value, index, array) {

    return
  value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

JavaScript Array.from()

The Array.from() method returns an Array object from:

• Any iterable object
• Any object with a length property

Example

let text = "ABCDEFG";

Array.from(text);

Array.from() has an optional parameter which allows you to execute a function on each element of the new array:

Example

const myNumbers = [1,2,3,4];

const myArr = Array.from(myNumbers, (x) => x * 2);

JavaScript Array keys()

The Array.keys() method returns an Array Iterator object with the keys of an array.

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];

const keys = fruits.keys();

for (let x of keys) {

  text += x + " ";
}

JavaScript Array entries()

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];

const f = fruits.entries();

for (let x of f) {

  document.getElementById("demo").innerHTML += x;
}

The entries() method returns an Array Iterator object with key/value pairs:

[0, "Banana"][1, "Orange"][2, "Apple"][3, "Mango"]

The entries() method does not change the original array.

JavaScript Array with() Method

ES2023 added the Array with() method as a safe way to update elements in an array without altering the original array.

Example

const months = ["Januar", "Februar", "Mar", "April"];

const myMonths = months.with(2, "March");

JavaScript Array Spread (...)

The ... operator expands an array into individual elements.

This can be used join arrays:

Example 1

const arr1 = [1, 2, 3];

const arr2 = [4, 5, 6];

const arr3 = [...arr1, ...arr2];

In the example above, ...arr1 expands arr1 into single elements, ...arr2 expands arr2 into single elements, and arr3 is constructed using ...arr1 and ...arr2.

Example 2

const q1 = ["Jan", "Feb", "Mar"];

const q2 = ["Apr", "May", "Jun"];

const q3 = ["Jul", "Aug", "Sep"];

const q4 = ["Oct", "Nov", "Des"];

const year = [...q1, ...q2, ...q3, ...q4];

The spread operator (...) can be used to copy an array:

Example 3

const arr1 = [1, 2, 3];

const arr2 = [...arr1];

The spread operator (...) can be used to pass arguments to a function:

Example 4

const numbers = [23,55,21,87,56];

let minValue = Math.min(...numbers);

let maxValue = Math.max(...numbers);

JavaScript Array Rest (...)

The rest operator (...) allows us to destruct an array and collect the leftovers:

Examples

let a, rest;

const arr1 = [1,2,3,4,5,6,7,8];

[a, ...rest] = arr1;