#function constructor

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); });

In some rare cases it is necessary to specify the names of a variable length list of arguments. Unfortunately, in JavaScript like in most languages there is no easy way to define a variable of these. If you use eval:

var args = 'a, b, c, doOperation', func; eval(' func = function(' + args + '){' + 'return (doOperation) ? a + (b * c) : -1;' + '}'); console.log(func(1, 2, 3, true));// 7 console.log(func(1, 2, 3, false));// -1

But eval is one character away from evil, for more info. Fortunately, since they are objects, functions can be constructed with strings allowing a variable number of arguments:

var func = new Function('a', 'b', 'c', 'doOperation', 'return (doOperation) ? a + (b * c) : -1;'); console.log(func(1, 2, 3, true));// 7 console.log(func(1, 2, 3, false));// -1

Though making longer functions via strings can be annoying. To overcome this, A toString() on an existing function can be used. This copies the entire function including the function keyword and declaration syntax which is annoying to remove in a way compatible across multiple environments. So instead the string of a function can be wrapped in a way that makes it self executing:

var args = ['a', 'b', 'c', 'doOperation']; var body = function(){ return (doOperation) ? a + (b * c) : -1; } var func = new Function(args, 'return (' + body.toString() + ')()'); console.log(func(1, 2, 3, true));// 7 console.log(func(1, 2, 3, false));// -1

Anyways, I hope you found this post enjoyable and maybe even useful.