Array .includes() / indexOf() / lastIndexOf()

var result = array.includes(searchValue, startIndex);

var index = array.indexOf(searchValue, startIndex);

var index = array.lastIndexOf(searchValue, startIndex);

var members = ["item_a", "item_b", "item_c", "item_b", "item_d"];

console.log(members.includes("item_b")); // Outputs: true
console.log(members.includes("item_e")); // Outputs: false

console.log(members.indexOf("item_b")); // Outputs: 1
console.log(members.indexOf("item_e")); // Outputs: -1 (not found)

console.log(members.lastIndexOf("item_b")); // Outputs: 3

// Search starting from a given index.
console.log(members.indexOf("item_b", 2)); // Outputs: 3 (searches from index 2 onward)

// NaN handling differs between the two methods.
var arr = [1, NaN, 3];
console.log(arr.includes(NaN)); // Outputs: true
console.log(arr.indexOf(NaN)); // Outputs: -1 (indexOf cannot detect NaN)

var validStatuses = ["active", "inactive", "pending"];
var inputStatus = "pending";
if (validStatuses.includes(inputStatus)) {
	console.log(inputStatus + " is a valid status"); // Outputs 'pending is a valid status'.
}

// Permission check: Verify whether a user has a specific role.
var userRoles = ["viewer", "editor"];
console.log(userRoles.includes("admin")); // Outputs false — user does not have admin access.
console.log(userRoles.includes("editor")); // Outputs true — user has editor access.

true
false
1
-1
3
3
true
-1
pending is a valid status
false
true

array.includes() is the best choice when you only need to know whether a value exists. Because it returns a boolean directly, you can use it as an if condition without the verbose array.indexOf() !== -1 pattern. It was added in ES2016 and has the added benefit of correctly handling NaN.

array.indexOf() has been available since ES5 and is useful when you need to know the position of a value. Because NaN === NaN evaluates to false in JavaScript, array.indexOf() cannot detect NaN. Use array.includes() to check for the presence of NaN.

These methods use strict comparison (equivalent to ===), so values of different types will not match. Objects and arrays match only if they share the same reference. For advanced searches using a condition function, use array.find() / array.findIndex().