Node.js Professionals Alliance Berlin #1 Meetup: Generators

basic.js

var Foo = function * (c) {
        var a = [0, 1],
            b = 0;

        for (b = 0; b < c; ++b) {
        	a.push(a[0] + a[1]);
        	a.shift();

        	yield a[a.length - 1];
        }

        return a[0]
}

// initialize generator
var foo = Foo(12);

// kick off generator
var a = foo.next(); // { value: 1, done: false }

// using for .. of
for ( var a of foo ) {
	console.log(a);

	/*
		2
		3
		5
		8
		13
		21
		34
		55
		89
		144
		233
	*/
}

/*
	^^^^^^^^^^^^^^^^
	for .. of will directly use "value" of the result,
	BUT you won't be able to pass a parameter to .next
	nor will you be able to .throw from the iterator.
*/

basic2.js

// Math example

var FooMath = function * (){
	var a = 0;

	for(;;) {
		a = yield a;

		++a;
	}
}

/*
	The first .next will yield 0 (a) and when passed a value,
	the passed value will substitute the expression and resume
	at that exact location. Let's mutate "a".
*/

var fooMath = FooMath();

fooMath.next(); // { value: 0, done: false }
fooMath.next(100); // { value: 101, done: false }

complex1ParamAndThrow.js

// Hypothetical use in an application

/*
	We have a generator that obtains user data;
	we will inspect the return values and inject
	the info that is asked for by using .next
	and will throw errors in the generator-scope
	by using the .throw method.
*/

var db = {
	"kenansulayman": {
		name: "Kenan Sulayman",
		job: "Software Architect"
	},
	"homerdong": {
		name: "Homer Dong",
		job: "PHP veteran"
	}
}

var fooGetVal = function * (id){
	// user id is "id"

	var user = {
		id: id
	}

	try {
		user.name = yield {
			type: "name",
			id: id
		};
	} catch(e) {
		if ( e === "No such id" ) {
			return yield {
				type: "result",
				data: null
			};
		}
	}

	yield {
		type: "result",
		data: user
	};
}

// this will usually be in a loop:
var reaper = function (cb, myGetVal, param) {
	var retval = myGetVal.next(param).value; // { type: "name", id: "kenansulayman" }

	if ( retval.type === "name" ) {
		if ( db[retval.id] ) {
			param = db[retval.id].name;
		} else {
			myGetVal.throw("No such id");

			return cb(null);
		}
	}

	if ( retval.type === "result" ) {
		return cb(retval.data);
	}

	return reaper(cb, myGetVal, param)
};

var myGetVal = fooGetVal("kenansulayman");

	reaper(function (user) {
		console.log(user); // { id: 'kenansulayman', name: 'Kenan Sulayman' }
	}, myGetVal);

var myGetVal2 = fooGetVal("homerdong");

	reaper(function (user) {
		console.log(user); // { id: 'homerdong', name: 'Homer Dong' }
	}, myGetVal2);

var myGetVal3 = fooGetVal("peter");

	reaper(function (user) {
		console.log(user); // "null"; see line 32 - 35
	}, myGetVal3);

complex2Delegation.js

var foo = function * () {
	var a = 10;

	while (a--) yield a;
}

var foo2 = function * () {
	var a = 10;

	yield* foo(); // delegate to foo^

	while (a--) yield Math.random();
}

for ( var a of foo2() ) {
	console.log(a)
}

// Example output

/*
	9
	8
	7
	6
	5
	4
	3
	2
	1
	0
	0.31441035214811563
	0.932482686592266
	0.49652760615572333
	0.5199203079100698
	0.7944680366199464
	0.21947496198117733
	0.7085313643328846
	0.11945359944365919
	0.20332212862558663
	0.15203028940595686
*/