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

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indexOf(searchElement, fromIndex)



Element to locate in the array.

fromIndex Optional

The index to start the search at. If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array. Note: if the provided index is negative, the array is still searched from front to back. If the provided index is 0, then the whole array will be searched. Default: 0 (entire array is searched).

Return value

The first index of the element in the array; -1 if not found.


indexOf() compares searchElement to elements of the Array using strict equality (the same method used by the === or triple-equals operator).

Note: For the String method, see String.prototype.indexOf().


Using indexOf()

The following example uses indexOf() to locate values in an array.

const array = [2, 9, 9];
array.indexOf(2);     // 0
array.indexOf(7);     // -1
array.indexOf(9, 2);  // 2
array.indexOf(2, -1); // -1
array.indexOf(2, -3); // 0

Finding all the occurrences of an element

const indices = [];
const array = ['a', 'b', 'a', 'c', 'a', 'd'];
const element = 'a';
let idx = array.indexOf(element);
while (idx !== -1) {
  idx = array.indexOf(element, idx + 1);
// [0, 2, 4]

Finding if an element exists in the array or not and updating the array

function updateVegetablesCollection (veggies, veggie) {
  if (veggies.indexOf(veggie) === -1) {
    console.log(`New veggies collection is: ${veggies}`);
  } else {
    console.log(`${veggie} already exists in the veggies collection.`);

const veggies = ['potato', 'tomato', 'chillies', 'green-pepper'];

updateVegetablesCollection(veggies, 'spinach');
// New veggies collection is: potato,tomato,chillies,green-pepper,spinach
updateVegetablesCollection(veggies, 'spinach');
// spinach already exists in the veggies collection.


ECMAScript Language Specification
# sec-array.prototype.indexof

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See also