Find the Index
findIndex returns the index of the first item that matches.
If nothing matches, it returns -1.
JavaScript Tutorial
JavaScript findIndex()
findIndex returns the position of the first item that matches a condition.
It is useful when you need to update or remove items by index.
Why We Need It
Many updates require the index of an item. findIndex gives you that quickly.
It stops early, so it is efficient for lookups.
Syntax
array.findIndex((value, index, array) => condition)Basic Example
1. Find index
const users = [{ id: 1 }, { id: 2 }];
const idx = users.findIndex((u) => u.id === 2);
console.log(idx);Returns 1 for the item with id 2.
Real World Example
2. Not found
const nums = [1, 2, 3];
const idx = nums.findIndex((n) => n > 10);
console.log(idx); // -1-1 indicates no match.
Multiple Use Cases
Use for Updates
Once you have the index, you can update or remove the item.
This is useful in state updates and list editing.
Stop Early
Like find, it stops after the first match.
That makes it efficient for lookups.
More Examples
3. Update by index
const items = [{ id: 1, done: false }];
const idx = items.findIndex((i) => i.id === 1);
if (idx !== -1) items[idx].done = true;
console.log(items);Use the index to update a specific item.
4. Remove item
const list = ["a", "b", "c"];
const idx = list.findIndex((v) => v === "b");
if (idx !== -1) list.splice(idx, 1);
console.log(list);Remove an item by index.
Comparison
Without
let idx = -1;
for (let i = 0; i < users.length; i++) {
if (users[i].id === 2) {
idx = i;
break;
}
}With
const idx = users.findIndex((u) => u.id === 2);Common Mistakes and Fixes
Forgetting -1 case
Always check if index is -1 before using it.
Expecting the value
findIndex returns the position, not the item.
Using findIndex for multiple matches
Use filter or a loop to collect all matches.
Interview Questions
What does findIndex return if no match?
-1.
When should you use findIndex?
When you need the position to update or remove.
Does findIndex stop after first match?
Yes, it stops early.
Practice Problem
Practice: Find the index of the first negative number.
const nums = [3, -1, 4];
// TODO: find index of first negative
One Possible Solution
const nums = [3, -1, 4];
const idx = nums.findIndex((n) => n < 0);
console.log(idx);Frequently Asked Questions
What does findIndex return?
The index of the first match or -1.
When should I use find instead?
Use find when you need the element, not the index.
Does findIndex mutate the array?
No, it does not.
Try It Yourself
Try changing the array to see different indexes.