Prefix an integer with zeros

JavaScript

const prefixWithZeros = (n, length) => (n / Math.pow(10, length)).toFixed(length).substr(2)

// Or
const prefixWithZeros = (n, length) => `${Array(length).join('0')}${n}`.slice(-length)

// Or
const prefixWithZeros = (n, length) => String(n).padStart(length, '0')

TypeScript

const prefixWithZeros = (n: number, length: number): string => (n / Math.pow(10, length)).toFixed(length).substr(2)

// Or
const prefixWithZeros = (n: number, length: number): string => `${Array(length).join('0')}${n}`.slice(-length)

// Or
const prefixWithZeros = (n: number, length: number): string => String(n).padStart(length, '0')

Examples

prefixWithZeros(42, 5) // '00042'