W3cubDocs

Array.prototype.indexOf()

The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

Try it

Syntax

indexOf(searchElement)
indexOf(searchElement, fromIndex)

Parameters

searchElement: Element to locate in the array.
fromIndex Optional: The index to start the search at. If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array. Note: if the provided index is negative, the array is still searched from front to back. If the provided index is 0, then the whole array will be searched. Default: 0 (entire array is searched).

Return value

The first index of the element in the array; -1 if not found.

Description

indexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator).

Note: For the String method, see String.prototype.indexOf().

Examples

Using indexOf()

The following example uses indexOf() to locate values in an array.

const array = [2, 9, 9];
array.indexOf(2);     // 0
array.indexOf(7);     // -1
array.indexOf(9, 2);  // 2
array.indexOf(2, -1); // -1
array.indexOf(2, -3); // 0

Finding all the occurrences of an element

const indices = [];
const array = ['a', 'b', 'a', 'c', 'a', 'd'];
const element = 'a';
let idx = array.indexOf(element);
while (idx !== -1) {
  indices.push(idx);
  idx = array.indexOf(element, idx + 1);
}
console.log(indices);
// [0, 2, 4]

Finding if an element exists in the array or not and updating the array

function updateVegetablesCollection (veggies, veggie) {
  if (veggies.indexOf(veggie) === -1) {
    veggies.push(veggie);
    console.log(`New veggies collection is: ${veggies}`);
  } else {
    console.log(`${veggie} already exists in the veggies collection.`);
  }
}

const veggies = ['potato', 'tomato', 'chillies', 'green-pepper'];

updateVegetablesCollection(veggies, 'spinach');
// New veggies collection is: potato,tomato,chillies,green-pepper,spinach
updateVegetablesCollection(veggies, 'spinach');
// spinach already exists in the veggies collection.

Specifications

Specification
ECMAScript Language Specification # sec-array.prototype.indexof

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	Deno	Node.js
`indexOf`	1	12	1.5	9	9.5	3	≤37	18	4	10.1	1	1.0	1.0	0.10.0