Understanding indexOf() & lastIndexOf() array methods in Javascript | examples

indexOf()

The indexof() method returns the first index of the element found in the array. It returns a value of -1 if the element is not found. This method is helpful when you want to check if a given element is present in an array.

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

let numberIndex = numbers.indexOf(5);

console.log(numberIndex); // 4

If the element is not present, the method returns -1.

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

let numberIndex = numbers.indexOf(35);

console.log(numberIndex); // -1

Similarly, let us look at an example containing an array of strings.

let strings = ["One", "Two", "Three", "Four", "Five"];

let searchIndex = strings.indexOf("Two");

console.log(searchIndex); // 1

Note, the indexOf() looks for the exact value. Hence, it will not work if the string is case-sensitive.

let strings = ["One", "Two", "Three", "Four", "Five"];

let searchIndex = strings.indexOf("two");

console.log(searchIndex); // -1

lastIndexOf()

The lastIndexOf() method is similar to the indexOf() method except it returns the last index of the element found in a given array. It returns a -1 if the element is not present in the array.

let strings = ["One", "Two", "Three", "Four", "Five", "Two"];

let searchIndex = strings.lastIndexOf("Two");

console.log(searchIndex); // 5