Check if a character is a digit

JavaScript

const isDigit = (char) => char < 10

// Or
const isDigit = (char) => char.length === 1 && c >= '0' && c <= '9'

// Or
const isDigit = (char) => Boolean([true, true, true, true, true, true, true, true, true, true][char])

Examples

isDigit('a') // false
isDigit('abc') // false
isDigit(10) // false
isDigit('10') // false

isDigit('2') // true
isDigit(2) // true