JavaScript (ES2026) reference · Array Methods

JavaScript Array indexOf() Method

Return the first index at which a value is found (using ===), or -1 if it isn't in the array.

JavaScript (ES2026) Since ES5

Syntax

array.indexOf(searchElement, fromIndex)

Parameter	Description
`searchElement`	The value to look for.
`fromIndex` optional	Index to start from. Default: `0`

An index, or -1.

Find a positionTry it yourself

const letters = ["a", "b", "c", "b"];
console.log(letters.indexOf("b"), letters.indexOf("b", 2), letters.indexOf("z"));

Output

1 3 -1

Prefer includes() for yes/no checksTry it yourself

const items = ["x", "y"];
if (items.indexOf("y") !== -1) console.log("found");
if (items.includes("y")) console.log("found (clearer)");

Output

found
found (clearer)

Version	Change
JavaScript ES5	Added

array.lastIndexOf() · Return the last index at which a value is found (using ===), searching backwards, or -1 if it isn't there.
array.includes() · Return true if the array contains a value, using SameValueZero equality (so it finds NaN, unlike indexOf).
array.findIndex() · Return the index of the first element that passes a test function, or -1 if none does.
JavaScript Arrays · tutorial
JavaScript Array Methods · tutorial