Array.prototype.indexOf()

Overview.

The Array.prototype.indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

Syntax

array.indexOf(searchElement[, fromIndex])

Parameters

• searchElement (required): The element to search for in the array.
• fromIndex (optional): The index to start the search from. If not specified, the search starts from the beginning of the array.

return value

The index of the first occurrence of the specified element in the array, or -1 if it is not found.

Examples

const numbers = [2, 5, 9, 2];
console.log(numbers.indexOf(5)); // Output: 1
console.log(numbers.indexOf(2)); // Output: 0
console.log(numbers.indexOf(7)); // Output: -1

Use Cases

Checking if an element exists in an array

You can use indexOf() to check whether a specific element exists in an array or not. If the method returns -1, the element does not exist in the array. If the method returns a non-negative integer, the element exists in the array.

const fruits = ["apple", "banana", "orange", "pear"];
const searchFruit = "banana";

if (fruits.indexOf(searchFruit) !== -1) {
  console.log(`${searchFruit} exists in the array.`);
} else {
  console.log(`${searchFruit} does not exist in the array.`);
}

Removing an element from an array

You can use indexOf() along with the Array.prototype.splice() method to remove an element from an array.

const fruits = ["apple", "banana", "orange", "pear"];
const elementToRemove = "orange";

if (fruits.indexOf(elementToRemove) !== -1) {
  fruits.splice(fruits.indexOf(elementToRemove), 1);
  console.log(
    `The element ${elementToRemove} has been removed from the array.`
  );
} else {
  console.log(`The element ${elementToRemove} does not exist in the array.`);
}

console.log(fruits); // Output: ['apple', 'banana', 'pear']

Notes

• indexOf() compares the searched element against each element of the array using strict equality (===).
• If you want to find the last index of an element, you can use the Array.prototype.lastIndexOf() method.
• If you want to check whether an element is present in an array, you can use the Array.prototype.includes() method.