JavaScript Array indexOf()

 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

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()