Find All Occurrence Indices in a JavaScript Array

Here are the different methods to find the index of all occurrences of a specific element in an array

1. Using While Loop

Use a while loop to iterate through the array, and if an element matches the target, push its index into the array. Finally, return the array of indexes.

let a = ["GFG", "Geeks", "Portal", "Computer Science", "GFG", "GFG", "Geek"];
let elm = "GFG";

let ind = [],
    i = -1;
while ((i = a.indexOf(elm, i + 1)) !== -1) {
    ind.push(i);
}
console.log(ind);

[ 0, 4, 5 ]

In this example

• The while loop iterates through the array, finding the index of all element occurrences using indexOf().
• The result is stored in the ind array, which holds the indices of each occurrence of elm in a.

2. Using reduce() Method

To find all indices of a specified element using reduce(), iterate through the array and push the index into a result array if the element matches the given value. Finally, return the array of indices.

let a = ["GFG", "Geeks", "Portal", "Computer Science", "GFG", "GFG", "Geek"];
let elm = "GFG";

let index = a.reduce((ind, el, i) => {
    if (el === elm) ind.push(i);
    return ind;
}, []);

console.log(index);

[ 0, 4, 5 ]

In this example

• reduce(): Iterates through the array, pushing the index i into the ind array whenever the element el matches elm.
• The result is directly logged.

3. Using forEach() Method

The arr.forEach() method calls the provided function once for each element of the array.

let a = [1, 2, 3, 2, 4, 2, 5];
let tar = 2;

let ind = [];
a.forEach((val, index) => {
    if (val === tar) ind.push(index);
});
console.log(ind);

[ 1, 3, 5 ]

In this example

• forEach(): Loops through the array and pushes the index into the ind array whenever the value matches tar.
• The result is directly logged.

4. Using map() and filter() Method

The map() and filter() methods to find the index of all occurrences of elements in an array using JavaScript.

let a = ["GFG", "Geeks", "Portal", "Computer Science", "GFG", "GFG", "Geek"];
let elm = "GFG";

let arr = a
    .map((element, index) => (element === elm ? index : -1))
    .filter((index) => index !== -1);

console.log(arr);

[ 0, 4, 5 ]

In this example

• map(): Creates an array of indices where the element equals elm or -1 if it doesn't match.
• filter(): Removes -1 values, leaving only the indices where elm was found.

6. Using for...of Loop with entries() Method

Use the for...of loop in combination with the entries() method to iterate over the array with both the index and value. If the value matches the target, we push the index to the results array.

let a = [1, 2, 3, 2, 4, 2, 5];
let tar = 2;

let ind = [];
for (const [index, val] of a.entries()) {
    if (val === tar) ind.push(index);
}

console.log(ind);

[ 1, 3, 5 ]

In this example

• for...of with entries(): Iterates through the array, giving both the index and value. If the value matches tar, the index is pushed into ind.
• The result is logged directly.