Sometimes you don't need the item itself, you need to know where it sits in the array. Maybe you want to update it, remove it, or just check that it exists. That's the job of findIndex(). It searches with a condition and gives you back the position of the first match. This guide covers how it works and the traps worth knowing about.
 

Table of Contents

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What findIndex() Does

Returns the position of the first match: it gives you a number, the index, not the item.

Stops at the first match: later items never get checked.

Takes a condition: so it can handle logic, like "greater than 10" or "has this ID".

Leaves the original array alone: it only reads through it.

let numbers = [5, 12, 8, 20, 3];
let index = numbers.findIndex(n => n > 10);

console.log(index); // 1

 

Syntax and Parameters

array.findIndex(function (item, index, array) {
  return condition;
});

item: the current element being checked.

index: its position in the array.

array: the full array. You'll rarely need it.

The function must return true or false: that decides whether the item counts as a match.
 

Practical Examples

Finding the position of an object:

let users = [
  { id: 1, name: "Vi" },
  { id: 2, name: "Sam" },
  { id: 3, name: "Alex" }
];

let index = users.findIndex(u => u.id === 2);
console.log(index); // 1

Updating an item at that position:

let tasks = [
  { id: 1, title: "Study", done: false },
  { id: 2, title: "Gym", done: false }
];

let i = tasks.findIndex(t => t.id === 2);

if (i !== -1) {
  tasks[i].done = true;
}

Removing an item at that position:

let j = tasks.findIndex(t => t.id === 1);

if (j !== -1) {
  tasks.splice(j, 1);
}

 

Things to Remember

No match gives you -1: not undefined, and not an error.

console.log([1, 2, 3].findIndex(n => n > 100)); // -1

Check against -1 explicitly: a quick truthy check goes wrong, because index 0 is falsy.

let arr = [100, 200, 300];
let i = arr.findIndex(n => n === 100);

console.log(i);        // 0
console.log(!i);       // true, looks like "not found" but isn't
console.log(i !== -1); // true, the correct check

Only the first match counts: if several items pass, you only get one index.
 

Common Mistakes

Using if (!index) to check for not found: this treats a valid index of 0 as a miss.

Using splice with -1: splice(-1, 1) removes the last item, so always check for -1 first.

Forgetting to return a value: with curly braces, you need an explicit return, otherwise it always returns -1.
 

Conclusion

Frequently Asked Questions (FAQs)

find() returns the matching item. findIndex() returns its position.

2.Why does findIndex() return -1 when nothing matches?

Because it returns a position, and -1 is the usual way to say "not found" for an index.

3.Can findIndex() search an array of objects?

Yes. You can write a condition on any property, which indexOf() can't do.