render-mustache.js

function renderMustache(str, obj) {
  return Object.keys(obj).reduce(function (memo, key) {
    var value = obj[key];
    var regex = new RegExp('\\$\\{' + key + '\\}', 'g');

    return memo.replace(regex, value);
  }, str);
};

// usage
renderMustache('thing/${a}/stuff/${b}', { a:1, b:2 });
// > 'thing/1/stuff/2'