The indexOf()
method returns the first index of occurance of an array element, or -1 if it is not found.
Example
let languages = ["Java", "JavaScript", "Python", "JavaScript"];
// get the index of the first occurrence of "JavaScript"
let index = languages.indexOf("JavaScript");
console.log(index);
// Output: 1
indexOf() Syntax
The syntax of the indexOf()
method is:
arr.indexOf(searchElement, fromIndex)
Here, arr is an array.
indexOf() Parameters
The indexOf()
method takes in:
- searchElement - The element to locate in the array.
- fromIndex (optional) - The index to start the search at. By default, it is 0.
indexOf() Return Value
- Returns the first index of the element in the array if it is present at least once.
- Returns -1 if the element is not found in the array.
Note: indexOf()
compares searchElement
to elements of the Array using strict equality (similar to triple-equals operator or ===
).
Example 1: Using indexOf() method
var priceList = [10, 8, 2, 31, 10, 1, 65];
// indexOf() returns the first occurance
var index1 = priceList.indexOf(31);
console.log(index1); // 3
var index2 = priceList.indexOf(10);
console.log(index2); // 0
// second argument specifies the search's start index
var index3 = priceList.indexOf(10, 1);
console.log(index3); // 4
// indexOf returns -1 if not found
var index4 = priceList.indexOf(69.5);
console.log(index4); // -1
Output
3 0 4 -1
Notes:
- If fromIndex >= array.length, array is not searched and -1 is returned.
- If fromIndex < 0, the index is calculated backward. For example, -1 denotes the last element's index and so on.
Example 2: Finding All the Occurrences of an Element
function findAllIndex(array, element) {
indices = [];
var currentIndex = array.indexOf(element);
while (currentIndex != -1) {
indices.push(currentIndex);
currentIndex = array.indexOf(element, currentIndex + 1);
}
return indices;
}
var priceList = [10, 8, 2, 31, 10, 1, 65, 10];
var occurance1 = findAllIndex(priceList, 10);
console.log(occurance1); // [ 0, 4, 7 ]
var occurance2 = findAllIndex(priceList, 8);
console.log(occurance2); // [ 1 ]
var occurance3 = findAllIndex(priceList, 9);
console.log(occurance3); // []
Output
[ 0, 4, 7 ] [ 1 ] []
Example 3: Finding If Element exists else Adding the Element
function checkOrAdd(array, element) {
if (array.indexOf(element) === -1) {
array.push(element);
console.log("Element not Found! Updated the array.");
} else {
console.log(element + " is already in the array.");
}
}
var parts = ["Monitor", "Keyboard", "Mouse", "Speaker"];
checkOrAdd(parts, "CPU"); // Element not Found! Updated the array.
console.log(parts); // [ 'Monitor', 'Keyboard', 'Mouse', 'Speaker', 'CPU' ]
checkOrAdd(parts, "Mouse"); // Mouse is already in the array.
Output
Element not Found! Updated the array. [ 'Monitor', 'Keyboard', 'Mouse', 'Speaker', 'CPU' ] Mouse is already in the array.
Recommended Reading: JavaScript Array.lastIndexOf()