JavaScript Array indexOf()

The indexOf() method returns the first index of occurance of an array element, or -1 if it is not found.

Example

let languages = ["Java", "JavaScript", "Python", "JavaScript"];

// get the index of the first occurrence of "JavaScript" let index = languages.indexOf("JavaScript");
console.log(index); // Output: 1

indexOf() Syntax

The syntax of the indexOf() method is:

arr.indexOf(searchElement, fromIndex)

Here, arr is an array.


indexOf() Parameters

The indexOf() method takes in:

  • searchElement - The element to locate in the array.
  • fromIndex (optional) - The index to start the search at. By default, it is 0.

indexOf() Return Value

  • Returns the first index of the element in the array if it is present at least once.
  • Returns -1 if the element is not found in the array.

Note: indexOf() compares searchElement to elements of the Array using strict equality (similar to triple-equals operator or ===).


Example 1: Using indexOf() method

var priceList = [10, 8, 2, 31, 10, 1, 65];

// indexOf() returns the first occurance
var index1 = priceList.indexOf(31);
console.log(index1); // 3
var index2 = priceList.indexOf(10);
console.log(index2); // 0 // second argument specifies the search's start index
var index3 = priceList.indexOf(10, 1);
console.log(index3); // 4 // indexOf returns -1 if not found
var index4 = priceList.indexOf(69.5);
console.log(index4); // -1

Output

3
0
4
-1

Notes:

  • If fromIndex >= array.length, array is not searched and -1 is returned.
  • If fromIndex < 0, the index is calculated backward. For example, -1 denotes the last element's index and so on.

Example 2: Finding All the Occurrences of an Element

function findAllIndex(array, element) {
  indices = [];
var currentIndex = array.indexOf(element);
while (currentIndex != -1) { indices.push(currentIndex);
currentIndex = array.indexOf(element, currentIndex + 1);
} return indices; } var priceList = [10, 8, 2, 31, 10, 1, 65, 10]; var occurance1 = findAllIndex(priceList, 10); console.log(occurance1); // [ 0, 4, 7 ] var occurance2 = findAllIndex(priceList, 8); console.log(occurance2); // [ 1 ] var occurance3 = findAllIndex(priceList, 9); console.log(occurance3); // []

Output

[ 0, 4, 7 ]
[ 1 ]
[]

Example 3: Finding If Element exists else Adding the Element

function checkOrAdd(array, element) {
if (array.indexOf(element) === -1) {
array.push(element); console.log("Element not Found! Updated the array."); } else { console.log(element + " is already in the array."); } } var parts = ["Monitor", "Keyboard", "Mouse", "Speaker"]; checkOrAdd(parts, "CPU"); // Element not Found! Updated the array. console.log(parts); // [ 'Monitor', 'Keyboard', 'Mouse', 'Speaker', 'CPU' ] checkOrAdd(parts, "Mouse"); // Mouse is already in the array.

Output

Element not Found! Updated the array.
[ 'Monitor', 'Keyboard', 'Mouse', 'Speaker', 'CPU' ]
Mouse is already in the array.

Recommended Reading: JavaScript Array.lastIndexOf()

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