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) {
  return (
    a.match(
      RegExp('[' + b.replace(/([.?*+^$[\]\\(){}|-])/g, '\\$1') + ']', 'g')
    ) || []
  ).join('');
}