Array.prototype.findIndex(callback) returns the index of the first element that passes your test. No match → -1. It stops early and does not mutate the array (ES2015).
array.findIndex((element, index, array) => test)
first truthy match → that index (0-based)
no match → -1
stops at the first hit
find() → the element (or undefined)
indexOf(x) → index by === equality onlyCall array.findIndex. Start with this line.
array.findIndex(callback, thisArg?)| Call | What it does |
|---|---|
array.findIndex(callback, thisArg?) | Primary signature. |
array.findIndex(...) | Call with the arguments your case needs. |
Built into JavaScript. Call findIndex(...) on an array instance — no import required.
array.findIndex(callback, thisArg?)| Parameter | Description |
|---|---|
| callback | Required. Test function; truthy → return that index |
| index, array | Optional. Extra callback args when needed |
| Return Value | Description |
|---|---|
| * | Index ≥ 0, or -1 |
Get the position of the first number greater than 25.
const numbers = [10, 20, 30, 40, 50];
const index = numbers.findIndex((num) => num > 25);
console.log(index); // 2
console.log(numbers[index]); // 300.> 25.2.numbers[2] reads the matching value.-1 When Nothing MatchesAlways check for -1 before indexing into the array.
const numbers = [10, 20, 30, 40, 50];
const index = numbers.findIndex((num) => num > 100);
if (index === -1) {
console.log("No element greater than 100 found.");
} else {
console.log("Found at index:", index);
}100.indexOf.arr[-1] as a lookup—check first.Find where "Bob" sits—then you can update or remove that slot.
const users = [
{ id: 1, name: "Alice" },
{ id: 2, name: "Bob" },
{ id: 3, name: "Charlie" }
];
const index = users.findIndex((user) => user.name === "Bob");
console.log(index); // 1
console.log(users[index]); // { id: 2, name: "Bob" }indexOf cannot match by name.user.name === "Bob".splice, or similar.Array.prototype.findIndex() is part of ES2015 (ES6). It has been available in evergreen browsers for years and in Node.js since version 4.
Supported in Chrome 45+, Firefox 25+, Safari 7.1+, Edge 12+, and Node 4+. Not available in Internet Explorer.
Bottom line: Safe for modern web and Node targets. Polyfill only if you must support Internet Explorer.
Find the first match when scanning from the end of the array.
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