dynamic.js

var Dynamic = function() {
  var context = {}
  var defaults = {}

  function bind(name, val, fn) {
    try {
      var save = context[name];
      context[name] = val;
      var out = fn();
    }
    finally {
      context[name] = save;
    }

    return out;
  }

  function get(name) {
    if (typeof context[name] === 'undefined') {
      return defaults[name];
    }
    else {
      return context[name];
    }
  }

  function setDefault(name, val) {
    if (arguments.length > 1) defaults[name] = val;

    return defaults[name];
  }

  function contextCopy() {
    var out = {}

    for (var k in context) {
      if (!hasOwnProperty.call(context, k)) continue;
      out[k] = context[k];
    }

    return out;
  }

  function remember(fn) {
    var savedContext = contextCopy();
    return function() {
      try {
        var swap = context;
        context = savedContext;
        var out = fn.apply(null, arguments);
      }
      finally {
        context = swap;
      }

      return out;
    };
  }

  return {
    bind: bind,
    get: get,
    setDefault: setDefault,
    remember: remember
  }
}();