The findIndex() Method in JavaScript

If you want to find the first matching element from an array but need its index, use the findIndex() method. It returns the index of the first matching element. If nothing is found, it returns -1.

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Syntax

    array.findIndex(function(element, index, array) {
      // returns true/false
    });

Parameters

• element - the current element
• index - the current index
• array - the entire array

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Simple Example

    const users = [
      { id: 1, name: 'Anna' },
      { id: 2, name: 'David' },
      { id: 3, name: 'Levon' }
    ];
    
    const foundIndex = users.findIndex(user => user.id === 2);
    
    console.log(foundIndex); // 1

In this case, findIndex() returns the index of the 2nd element that matches id: 2.

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If Nothing is Found

    const result = users.findIndex(user => user.id === 5);
    console.log(result); // -1

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Comparison with find()

• find() returns the first matching element.
• findIndex() returns the index of the first matching element.

    const even = [1, 2, 3, 4, 5];

    const found = even.find(n => n % 2 === 0);      // 2
    const foundIndex = even.findIndex(n => n % 2 === 0); // 1

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How findIndex() Works Internally

When we call:

    [1, 2, 3].findIndex(fn);

This is equivalent to:

    Array.prototype.findIndex.call([1, 2, 3], fn);

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Implement Our Own myFindIndex() Function from Scratch

    function myFindIndex(array, callback) {
      for (let i = 0; i < array.length; i++) {
        if (callback(array[i], i, array)) {
          return i;
        }
      }
      return -1;
    }

Let's apply it:

    const items = [10, 20, 30, 40];

    const foundIndex = myFindIndex(items, function(item) {
      return item > 25;
    });
    
    console.log(foundIndex); // 2

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Add It as a Method to the Array Prototype

    Array.prototype.myFindIndex = function(callback) {
      for (let i = 0; i < this.length; i++) {
        if (callback(this[i], i, this)) {
          return i;
        }
      }
      return -1;
    };

Usage:

    const numbers = [5, 10, 15, 20];

    const foundIndex = numbers.myFindIndex(n => n > 10);
    console.log(foundIndex); // 2

Conclusion

• Use findIndex() when you need to find the index of the first matching element.
• Returns -1 if nothing is found.
• If you need the index, use findIndex().