react-date-picker/src/utils/leftPad.js

A carefully crafted date picker for React

export default (str) => {

  if (typeof str == 'string' && str.length < 2){
    return str.length?
      '0' + str:
      '00'
  }

  if (typeof str == 'number'){
    return str < 10?
      '0' + str:
      str + ''
  }

  return str
}