I want to know if we can implement a function which can block in Javascript. For example:
var username = getUserName(); //wait for user input username in a certain way
doSth(username);
I hope getUserName can block until a valid username is input by the user. I am wondering if we could use coroutine or other ways to achieve this goal. Thank you.
In the browser, the closest you are going to get to coroutines are the Python-like generators proposed in the Ecmascript 6 standard. However, only Firefox and the bleeding edge versions of NodeJs natively support them as of today.
Given that most browsers don't support coroutines (and won't for a long time because of old versions of IE), your best bet is using a pure-Javascript control-flow library (seee "async.js" or promises) or use some tool that compiles a dialect of Javascript extended with coroutines back into continuation-passing-style. There are a couple that use the ES6 syntax and some others that use different syntaxes (and there are also some non-JS languages compiling to Javascript)
Finally, to keep things complete, in Nodejs has a Fibers feature that is a bit similar to coroutines but its server-side-only so I don't think they are going to be very useful to you.
This isn't "blocking" but you could always use a callback
function getUsername(done) {
try {
var username = // some process that gets the username;
return done(null, username);
}
catch (err) {
return done(err);
}
}
getUsername(function(err, username) {
if (err) throw err;
console.log(username);
}
Whith the arriving of generators to javascript and the help of Aryn is possible to use coroutines and CSP(Communicating Sequential Processes) in Node.js or in the client-side.
Aryn + Promise example in Node.js:
function getUsername(id) {
return new Promise(function(resolve, reject) {
db.query('select username from user where id = ?', [id], function(r){
resolve(r[0].username)
})
})
}
// your coroutine
aryn.run(function*(){
var username = yield getUsername(id)
console.log(username)
})
Using Aryns drive function:
db = aryn.drive(require('db'))
var getUsername = aryn.def(function*(){
return yield aryn.receive(db.query('select username from user where id = ?', [id]))
})
aryn.run(function*(){
var username = yield getUsername(id)
console.log(username)
})
prompt()function.showModalDialog. I would not recommended doing this, though.