Introduction to indexOf method in JavaScript

The indexOf() method in JavaScript is used to find the index of the first occurrence of a specified value in an array. This method searches through the array from the specified start index and returns the index of the first occurrence of the specified value.

Syntax

The syntax of the indexOf() method is as follows:

array.indexOf(searchValue [, fromIndex])

The array parameter is the array to be searched, and the searchValue parameter is the value to be searched for. The optional fromIndex parameter is the index at which to begin the search. If it is not specified, the search will begin at the first element of the array.

Return value

• If the searchValue is found, indexOf() returns the index of the first occurrence of the value.
• If the searchValue is not found, indexOf() returns -1.

Example

const array1 = [2, 5, 9, 10, 20];
console.log(array1.indexOf(10)); // output: 3

const array2 = ['apple', 'banana', 'orange', 'peach'];
console.log(array2.indexOf('orange', 1)); // output: 2
console.log(array2.indexOf('grape')); // output: -1

In the first example, indexOf() returns the index of the element '10', which is 3.

In the second example, indexOf() starts the search from index 1 and returns the index of the element 'orange', which is 2. The third console.log() statement searches for the element 'grape', which is not found, and therefore returns -1.

Resources

For more information about the indexOf() method, you can refer to the official documentation on MDN.