Check for a string in another string.

Remember that methods are usually case-sensitive. To do case-insensitive searches, use .toLowerCase() first.

Check if string contains a value

const x = 'abc'

x.includes('b')
// true

Get index of a value

const x = 'abcdef'

x.indexOf('cd')
// 2
x.indexOf('z')
// -1

'Blue Whale'.indexOf('Blue') !== -1
// true

You’ll get the same result with .search():

x.search('cd')
// 2
x.search('z')
// -1

Count occurences

Based on MDN docs.

Use indexOf() to count occurrences of a letter in a string:

const str = 'To be, or not to be, that is the question.'
let count = 0
let position = str.indexOf('e')

while (position !== -1) {
  count++
  position = str.indexOf('e', position + 1)
}

console.log(count)
// 4

Contrast with Python and built-in method:

>>> x = 'abcabc'
>>> x.count('a')
2
>>> x.count('abc')
2