#function.apply

It can be very useful to track custom statistics on JavaScript functions. For example, tracking all the times console.log is called:

console.log = track(console.log); console.log('message1'); console.log('message2'); console.log('message3'); console.log(console.log.called);// 3

Since this is JavaScript, the implementation is very simple:

var track = function(tracking) { return function tracker() { // Must transform the arguments sent in into an array which // .apply sends as arguments to the function var args = Array.prototype.slice.call(arguments); tracking.apply(this, args); // tracker.called is only falsy, when 0 or undefined here tracker.called = (tracker.called || 0) + 1; } }

Of course most JS profilers (such as browser based ones) track function call counts, but the above can be modified to track any statistic.

The JavaScript with statement can be a useful shortcut for accessing object properties:

var mailBox = { messages: { message1: 'hello', message2: 'I could not think of a message' } }; with(mailBox.messages){ console.log(message1); console.log(message2); }

Unfortunately, it is not very cross browser and will be phased out of JavaScript. So it is better to make a custom one:

var with_ = function(obj, callback) { var args = [] for (var property in obj){ args.push(obj[property]); } // Apply sets the context for the function and passes it args // specified by the array. callback.apply(null, args); }; with_(mailBox.messages, function(message1, message2){ console.log(message1); console.log(message2); });

But this still involves specifying function parameters, By defining a wrapper function it is possible to get rid of this:

var with_ = function(obj, callback) { var names = [], values = []; for (var property in obj){ names.push(property); values.push(obj[property]); } // Create wrapper that runs the original function with the obj properties // as parameters var wrapper = Function.apply(null, names.concat('(' + callback + ')();')); // Execute the wrapper with the values of the object wrapper.apply(null, values); }; with_(mailBox.messages, function(){ console.log(message1); console.log(message2); });