JavaScript Array indexOf and lastIndexOf: Locating an Element in an Array

Summary: in this tutorial, we will show you how to use the JavaScript array indexOf() and lastIndexOf() methods to find the position of an element in an array.

Introduction to the JavaScript array indexOf() method

To find the position of an element in an array, you use the indexOf() method. This method returns the index of the first occurrence the element that you want to find, or -1 if the element is not found.

The following illustrates the syntax of the indexOf() method.

Array.indexOf(searchElement, fromIndex)
Code language: JavaScript (javascript)

As shown above, the indexOf() method accepts two named arguments.

1. The searchElement argument is the element that you want to find in the array.
2. The fromIndex is an array index at which the function starts the search.

The fromIndex argument can be a positive or negative integer. If the fromIndex argument is negative, the indexOf() method starts searching at array’s length plus fromIndex.

In case you omit the fromIndex argument, the indexOf() method starts searching from the begining of the string.

Notice that the indexOf() method uses the strict equality comparison algorithm that is similar to the triple-equals operator (===) when comparing the searchElement with the elements in the array.

The JavaScript array indexOf() method examples

Suppose, you have an array scores that consists of six numbers as follows:

var scores = [10, 20, 30, 10, 40, 20];Code language: JavaScript (javascript)

The following example uses the indexOf() method to find the elements in the scores array:

console.log(scores.indexOf(10)); // 0
console.log(scores.indexOf(30)); // 2
console.log(scores.indexOf(50)); // -1
console.log(scores.indexOf(20)); // 1Code language: JavaScript (javascript)

And the following example uses the fromIndex() with the negative values:

console.log(scores.indexOf(20,-1)); // 5 (fromIndex = 6+ (-1) = 5)
console.log(scores.indexOf(20,-5)); // 1 (fromIndex = 6+ (-5) = 1)Code language: JavaScript (javascript)

Assuming that you have the following array of objects, where each object has two properties: name and age.

var guests = [
    {name: 'John Doe', age: 30},
    {name: 'Lily Bush', age: 20},
    {name: 'William Gate', age: 25}
];Code language: JavaScript (javascript)

The following statement returns -1 even though the first element of the guests array and the searchElement have the same values in the name and ages properties. This is because they are two different objects.

console.log(guests.indexOf({
    name: 'John Doe',
    age: 30
})); // -1Code language: JavaScript (javascript)

Sometimes, you want to find the indices of all occurrences of an element in an array. The following find() function uses the indexOf() method to do so.

function find(needle, haystack) {
    var results = [];
    var idx = haystack.indexOf(needle);
    while (idx != -1) {
        results.push(idx);
        idx = haystack.indexOf(needle, idx + 1);
    }
    return results;
}Code language: JavaScript (javascript)

The following example uses the find() function above to return an array of positions of the number 10 in the scores array.

console.log(find(10,scores)); // [0, 3]Code language: JavaScript (javascript)

JavaScript array lastIndexOf() method

The Array type has another method called lastIndexOf() that provides the similar functionality to the indexOf() method.

The following illustrates the syntax of the lastIndexOf() method:

Array.lastIndexOf(searchElement[, fromIndex = Array.length - 1])Code language: JavaScript (javascript)

The lastIndexOf() method returns the index of the last occurrence of the searchElement in the array. It returns -1 if it cannot find the element.

Different from the indexOf() method, the lastIndexOf() method searches for the element backward, starting at fromIndex.

The following statements return the last indices of the number 10 and 20 in the scores array.

console.log(scores.lastIndexOf(10));// 3
console.log(scores.lastIndexOf(20));// 5
Code language: JavaScript (javascript)

Because the number 50 is not in the scores array, the following statement returns -1.

console.log(scores.lastIndexOf(50));// -1Code language: JavaScript (javascript)

In this tutorial, you have learned how to use the JavaScript array indexOf() and lastIndexOf() methods to locate an element in the array.