JS promise/async clarification questions

Question

I've read about promise some document's and I've some basic question becouse some time I read that this is true and sometimes not.

I've two question's/clarification

1. if every function in Java script can call with promise (by using then) or I should return some promise object from the function i.e. define it differently by maybe add some Q to the return function ?
2. I saw that there is option to chain promises and also do Q.all what is the different between of them

An example will be very helpful

Answer 1

1. if every function in Java script can call with promise (by using then) or I should return some promise object from the function i.e. define it differently by maybe add some Q to the return function ?

Promises use return values. If a function does not return a promise - chaining it to another promise will not wait for anything. The way a chain (or an aggregate like .all) knows when a function is done is by using the return value.

function fn1(){
    setTimeout(function(){ console.log("Hi"); }, 1000);
}
Promise.resolve.then(function(){ // start empty ES6 promise chain
    return fn1();
}).then(function(){
    console.log("Bye"); // this will log "Bye Hi" because it did not wait.
});

The correct way would be to promisify it:

function fn2(){ // in Angular which you tagged, this is similar to `$timeout`
    return new Promise(function(resolve, reject){
        setTimeout(function(){ console.log("Hi"); resolve(); }, 1000);
    });
}
Promise.resolve.then(function(){ // start empty ES6 promise chain
    return fn2();
}).then(function(){
    console.log("Bye"); // this will log "Bye Hi" because it did not wait.
});

2. I saw that there is option to chain promises and also do Q.all what is the different between of them

Q.all or Promise.all in ES6 is parallel, chaining promises is sequential:

Promise.all([fn1(), fn2(), fn3]).then(function(){
    // fn1 fn2 and fn3 complete at an arbitrary order
    // code here executes when all three are done
});

fn1().then(fn2).then(fn3).then(function(){
   // fn1 executes, when it is done fn2, when it is done fn3,
   // then code here
});

3. There is some tool or some way to verify that the promise was chaind OK? becouse when I try forgot by mistake the return statement in some chain function which can harm the process.

Yes, Spion wrote a tool for this a while ago, it is called thenlint and you can find it here. The description says:

A lint for promises that checks for possible Promise.then usage errors.