indexOf

indexOf() is a handy little JavaScript method that returns the first index (position) of an item within an array or a string. For example:

Array -

const catStory = ['The', 'Cat', 'Is', 'Good', 'The', 'Cat', 'Is', 'Fluffy']
const firstCatOccurrence = 'Cat'

const firstCatPosition = catStory.indexOf(firstCatOccurrence)

console.log(firstCatPosition) // 1

Notes:

• indexOf() is case sensitive. Searching for 'cat' will return -1.
• Numbers can be searched as well, e.g. [1, 2, 3]

Or:

String -

const catStory = 'Pickles the cat slept in twelve different places today'
const catOccurrence = 'cat'

const catPosition = catStory.indexOf(catOccurrence)

console.log(catPosition) // 12

If an occurrence is not found, -1 will be returned. This is because the indexOf() method was written a long time ago, and back then it was thought of as sensible to always return a number. Methods such as match() and includes() were written later and return null and false when no occurrence is found. Therefore, to check whether an occurrence exists in your string (or array):

'Kitty Litter'.indexOf('Lit') !== -1 // true
'Kitty Litter'.indexOf('Kittty') !== -1 // false

See more from MDN about String.prototype.indexOf() here and Array.prototype.indexOf() here.