I was wondering if someone could assist in explaining something. I am learning JavaScript and am working my way through the Squiffy code. This function:

String.prototype.format = function() {
var args = arguments;
return this.replace(/{(\d+)}/g, function(match, number) {
return typeof args[number] != 'undefined'
? args[number]
: match
;
});
};

What I believe I understand is that this is a method added to the String prototype that assigns the arguments object which delivers the arguments in an 'array-esque' style where you can use index to reference but it is not an array. It then looks at the string and replaces everything between the { } with the unnamed function. Within the function I understand that it is basically returns the result of an 'if then else' statement'.

What I am struggling with is where the values of (match, number) originate from? If I console them out they are {0} and (0).

I know I am missing something at a pretty basic level and would appreciate if someone could point me in the right direction.

That code is actually from here: http://stackoverflow.com/questions/6104 ... ing-format

It's a JavaScript implementation of something like string.Format which exists in C#. It makes it easier to interpolate strings. For example, if I wanted to greet somebody by their first and last name, and tell them their age, normally I would have to do something like this...

var greeting = "Hello " + firstName + " " + lastName + ", you are " + age + ".";

By adding this "format" function to strings, I can do it like this:

var greeting = "Hello {0} {1}, you are {2}".format(firstName, lastName, age);

The way it works is: the variable "args" is an array of all arguments passed to the format function. So in the example above, the array is [firstName, lastName, age]. Then it calls this.replace with a regular expression that finds digits inside braces. It runs a function for every occurrence, and this function pulls the relevant value out of the arguments array.

The latest version of JavaScript allows for template strings (http://tc39wiki.calculist.org/es6/template-strings/), so in the future we'll be able to do this easily without having to create our own format function.