Sequential programming for node.js, end of callback hell.
Simple, straightforward abstraction over Fibers.
By using wait.for, you can call any nodejs standard async function in sequential/Sync mode, waiting for result data, without blocking node's event loop (thanks to fibers)
A nodejs standard async function is a function in which the last parameter is a callback: function(err,data)
Advantages:
I've developed a version based on JavaScript upcoming ES6-Harmony generators. It's not based on node-fibers. Surprisingly, ES6 based implementation of wait.for(asyncFn) is almost a no-op, you can even completely omit it. Warning: Bleeding edge. Check [Wait.for-ES6] (https://github.com/luciotato/waitfor-ES6)
npm install wait.for
You need to be in a Fiber to be able to use wait.for. The ideal place to launch a fiber is when a request arrives, to handle it:
var server = http.createServer(
function(req, res){
console.log('req!');
wait.launchFiber(handler,req,res); //handle in a fiber, keep node spinning
}).listen(8000);
then,at function handler(req,res) and every function you call from there, you'll be able to use wait.for(ayncFn...
var wait = require('wait.for');
function anyStandardAsync(param, callback){
setTimeout( function(){
callback(null,'hi '+param);
}, 5000);
};
function testFunction(){
console.log('fiber start');
var result = wait.for(anyStandardAsync,'test');
console.log('function returned:', result);
console.log('fiber end');
};
console.log('app start');
wait.launchFiber(testFunction);
console.log('after launch');
var wait = require('wait.for');
var express = require('express');
var app = express();
// in a Fiber
function handleGet(req, res){
res.send( wait.for(fs.readFile,'largeFile.html') );
}
app.get('/', function(req,res){
wait.launchFiber(handleGet, req, res); //handle in a fiber, keep node spinning
});
app.listen(3000);
see cradle example
var wait=require('wait.for');
// launch a new fiber
wait.launchFiber(my_sequential_function, arg,arg,...)
// in a fiber.. We can wait for async functions
function my_sequential_function(arg,arg...){
// call async_function(arg1), wait for result, return data
var myObj = wait.for(async_function, arg1);
// call myObj.querydata(arg1,arg2), wait for result, return data
var myObjData = wait.forMethod(myObj,'queryData', arg1, arg2);
console.log(myObjData.toString());
}
wait.for expects standardized callbacks. A standardized callback always returns (err,data) in that order.
A solution for the sql.query method and other non-standard callbacks is to create a wrapper function standardizing the callback, e.g.:
connection.prototype.q = function(sql, params, stdCallback){
this.query(sql,params, function(err,rows,columns){
return stdCallback(err,{rows:rows,columns:columns});
});
}
usage:
try {
var result = wait.forMethod(connection, "q", options.sql, options.params);
console.log(result.rows);
console.log(result.columns);
}
catch(err) {
console.log(err);
}
e.g.: node-sqlite3's database.prepare
var sqlite3 = require('sqlite3').verbose();
var db = new sqlite3.Database(':memory:');
db.prototype.prep = function(sql, stdCallback){
var stmt = this.prepare(sql, function(err){
return stdCallback(err, stmt);
});
}
var stmt = wait.forMethod (db, 'prep', "INSERT OR REPLACE INTO foo (a,b,c) VALUES (?,?,?)");
DNS testing, using pure node.js (a little of callback hell):
var dns = require("dns");
function test(){
dns.resolve4("google.com", function(err, addresses) {
if (err) throw err;
for (var i = 0; i < addresses.length; i++) {
var a = addresses[i];
dns.reverse(a, function (err, data) {
if (err) throw err;
console.log("reverse for " + a + ": " + JSON.stringify(data));
});
};
});
}
test();
THE SAME CODE, using wait.for (sequential):
var dns = require("dns"), wait=require('wait.for');
function test(){
var addresses = wait.for(dns.resolve4,"google.com");
for (var i = 0; i < addresses.length; i++) {
var a = addresses[i];
console.log("reverse for " + a + ": " + JSON.stringify(wait.for(dns.reverse,a)));
}
}
wait.launchFiber(test);
using pure node.js (a callback hell):
var db = require("some-db-abstraction");
function handleWithdrawal(req,res){
try {
var amount=req.param("amount");
db.select("* from sessions where session_id=?",req.param("session_id"),function(err,sessiondata) {
if (err) throw err;
db.select("* from accounts where user_id=?",sessiondata.user_ID),function(err,accountdata) {
if (err) throw err;
if (accountdata.balance < amount) throw new Error('insufficient funds');
db.execute("withdrawal(?,?)",accountdata.ID,req.param("amount"), function(err,data) {
if (err) throw err;
res.write("withdrawal OK, amount: "+ req.param("amount"));
db.select("balance from accounts where account_id=?", accountdata.ID,function(err,balance) {
if (err) throw err;
res.end("your current balance is " + balance.amount);
});
});
});
});
}
catch(err) {
res.end("Withdrawal error: " + err.message);
}
}
Note: The above code, although it looks like it will catch the exceptions, it will not. Catching exceptions with callback hell adds a lot of pain, and i'm not sure if you will have the 'res' parameter to respond to the user. If somebody like to fix this example... be my guest.
THE SAME CODE, using wait.for (sequential logic - sequential programming):
var db = require("some-db-abstraction"), wait=require('wait.for');
function handleWithdrawal(req,res){
try {
var amount=req.param("amount");
sessiondata = wait.forMethod(db,"select","* from session where session_id=?",req.param("session_id"));
accountdata = wait.forMethod(db,"select","* from accounts where user_id=?",sessiondata.user_ID);
if (accountdata.balance < amount) throw new Error('insufficient funds');
wait.forMethod(db,"execute","withdrawal(?,?)",accountdata.ID,req.param("amount"));
res.write("withdrawal OK, amount: "+ req.param("amount"));
balance = wait.forMethod(db,"select","balance from accounts where account_id=?", accountdata.ID);
res.end("your current balance is " + balance.amount);
}
catch(err) {
res.end("Withdrawal error: " + err.message);
}
}
Note: Exceptions will be catched as expected. db methods (db.select, db.execute) will be called with this=db
wait.launchFiber(genFn,param1,param2)
starts executing the function genFn
as a fiber-generator until a "yield" (wait.for) is found, then wait.launchFiber
execute the "yielded" value (a call to an async function), and links generator's "next" with the async callback(err,data), so when the async finishes and the callback is called, the fiber/generator "continues" after the var x =wait.for(...)
.
Note: must be in a Fiber
wait.parallel.launch expects an array of [[func,arg,arg..],[func,arg,arg..],...] and then launches a fiber for each function call, in parallel, and waits for all the fibers to complete.
The functions to be called should not be async functions.
Each called sync function will be executed in it's own fiber, and this sync function should/can use data=wait.for(..)
internally in order to call async functions.
-launchs a fiber for each func
-the fiber does resultArray[index] = func.apply(undefined,args)
Note: must be in a Fiber
Note: must be in a Fiber
Parallel Usage Example: see: