double-char.js

// https://www.codewars.com/kata/56b1f01c247c01db92000076/train/javascript

function doubleChar(str) {
  let result = "";
  // loop over the string
  for(let i = 0; i < str.length; i++) {
    // get the character that we're at for this looping
    const character = str[i];
    // add the doubled character to the result
    result += character + character;
  }
  return result;
}

fancy-double-char.js

// The fancy map way :)
function doubleChar(str) {
  return str.split('').map(character => character + character).join('')
}