templish/index.js

var regStr = "{{(.+)}}";
var localRegex = new RegExp(regStr);
var globalRegex = new RegExp(regStr, "g");

function expectType(type, val) {
	if (typeof val !== type)
		throw new TypeError("Expected "+type+", got "+typeof val);
}

function expectInstance(obj, val) {
	if (!(val instanceof obj)) {
		var regex = /function (.+)\(/;
		var expectedName = obj.toString().match(regex)[1];

		try {
			var realName = val.constructor.toString().match(regex)[1];
		} catch (err) {
			var realName = "undefined";
		}

		throw new TypeError("Expected "+expectedName+", got "+realName);
	}
}

function hasMultipleStatements(str) {
	return (str.indexOf(";") !== -1)
}

function Func(args) {
	return Function.apply(this, args);
}
Func.prototype = Function.prototype;

function Templish(str, vals) {
	expectType("string", str);
	expectInstance(Array, vals);

	str = str.replace(/\s+/g, " ").replace(/}}/g, "}}\n");
	this.string = str;
	this.vals = vals;
	this.subs = [];

	var matches = str.match(globalRegex);

	if (!matches)
		return;

	matches.forEach(function(m) {
		var str = m.match(localRegex)[1];

		//Add user defined arguments
		var args = [];
		vals.forEach(function(v) {
			args.push(v);
		});

		//We also need a callback function, for async things
		args.push("cb");

		//We want to automatically return if there's only one statement
		if (hasMultipleStatements(str))
			args.push(str);
		else
			args.push("return ("+str+")");

		this.subs.push({
			func: new Func(args),
			match: m+"\n"
		});
	}.bind(this));
}

Templish.prototype.exec = function(vals, cb) {
	expectInstance(Object, vals);
	expectType("function", cb);

	var str = this.string;

	//Prepare generic arguments
	var args = [];
	for (var i in vals) {
		var index = this.vals.indexOf(i);

		if (index === -1)
			throw new Error("Error: argument "+i+" doesn't exist");

		args[index] = vals[i];
	}

	var numArgs = args.length;

	//Keep track of how many callbacks we're waiting for
	var cbsRemaining = this.subs.length;

	//Replace
	this.subs.forEach(function(sub, i) {

		//Create a callback function for each substitute
		args[numArgs] = function(err, res) {
			if (err)
				throw err;

			str = str.replace(sub.match, res);

			//If this is the last callback to be executed,
			//call back with the resulting string
			if (cbsRemaining <= 0)
				cb(null, str);
			cbsRemaining -= 1;
		}

		var ret = sub.func.apply(null, args);

		//If the substitute's function returns anything, we use that directly,
		//but if it doesn't, we wait for a callback
		if (ret !== undefined) {
			str = str.replace(sub.match, ret);
			if (cbsRemaining <= 0)
				cb(str);
			cbsRemaining -= 1;
		}
	});

	//If there's no asynchronous functions to wait for,
	//just call back immediately
	if (cbsRemaining === 0)
		return cb(null, str);
}

module.exports = Templish;