a.md

Write a function f(a, b) which takes two strings as arguments and returns a string containing only the characters found in both strings, in the order that they appeared in a.

Original implementation found here: https://github.com/blakeembrey/code-problems/blob/master/common-string-filter/index.js

f.js

function f(a, b) {

  a = a.split('');    
  b = Array.prototype.slice.call(arguments, 1).join('').split('');

  return a.filter(function(val, i, arr) {
    return ~b.indexOf(val) && a.indexOf(val) === i
  }).join('')
}

Nice! Just updated mine to include any characters. Only issue with my approach is the returned string is not a unique set of characters, it may contain repeats in cases of, e.g. f('aa', 'a')