Does it mutate 😱

Array.prototype.indexOf

No mutation

Description

The indexOf() method of Array instances returns the first index at which a given element can be found in the array, or -1 if it is not present.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf

Example

const beasts = ['ant', 'bison', 'camel', 'duck', 'bison'];

console.log(beasts.indexOf('bison'));
// Expected output: 1

// Start from index 2
console.log(beasts.indexOf('bison', 2));
// Expected output: 4

console.log(beasts.indexOf('giraffe'));
// Expected output: -1