DailyJS

Using Vars to Shortcut Namespaces

19 Mar 2010 | By Alex Young | Tags codereview techniques

When I was researching the last turing.js framework article I came across a simple technique for namespace shortcuts in TJ Holowaychuk’s jspec. I haven’t noticed it used very often, so I thought DailyJS hackers may find it interesting.

;(function() {
  MyClass = {
    map: function(object, callback) {
      var items = [];
      each(object, function(i) { items.push(callback.apply(this, arguments)); });
      return items;
    },

    each: function(object, callback) {
      for (var i = 0; i < object.length; i++) {
        callback(i);
      }
    }
  }

  var methods = 'each map'.split(/ /);
  while (methods.length) { eval('var ' + methods[0] + ' = MyClass.' + methods.shift()); }
})();

// Map can refer to each without dereferencing
result = MyClass.map([1, 2, 3], function(i) {
  return i + 1;
});

print(result);
// 1,2,3

Ignore the implementations of each and map and look at the loop after MyClass.

What happens here is a list of methods are mapped to vars so they can be referenced without their containing object name.

  1. MyClass is declared globally
  2. Methods within the containing anonymous function can refer to hand-picked “utility” methods without referring to MyClass
  3. These methods are declared as local variables using eval
  4. The while loop iterates over each method name

Notice that the loop refers to each method using methods[0], then methods.shift() when it’s no longer needed, which keeps the loop control simple.

This is useful if you want to cut down code inside a library. It’s safe because the shortcuts are bound to the anonymous function, but novices may find navigating the code difficult. Tracking down where each comes from in a big project might be difficult, so avoid this technique if you’re working on a large project with colleagues with mixed skills.

This is flexible way of achieving similar results to with:


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