CODEWITHSUNDEEP
javascriptnode.jsasynchronouspromise
Ramesh Pareek
Ramesh Pareek

Reputation: 1669

Why this Javascript promise does not work?

I am Learning about promises.

app.get('/message',function(req, res){    
    var promise = new Promise(function(resolve, reject){
        resolve("hi");
    });
    promise.then(function(message){
       res.json(message);
    })

});

This works good. Though This is too simple. To write something 'lengthy' I moved the code out of app.get() and tried to return the message from the external function... like this: 

    app.get('/message',function(req, res){ 
      var message = message(); // I also tried wrapping this in promise and calling `res.json` in `promise.then()` but no luck
       res.json(message);
    });

    function message(){    
        var promise = new Promise(function(resolve, reject){
            resolve("hi");
        });
        promise.then(function(message){
           return message;
        })

    }

So why doesn't the return statement in the message() function return the message ? and what's the best practice to move such promising code out of my route functions?

Upvotes: 1

Views: 97

Answers (4)

palaѕн
palaѕн

Reputation: 73906

You can return promise from your message function and use async/await on it like:

app.get('/message', async function(req, res){
    var msg = await message();
    res.json(msg);
});
function message() {    
    return new Promise(function(resolve, reject){
        resolve("hi");
    });
}

Like:

function message() {
  return new Promise(function(resolve, reject) {
    setTimeout(resolve, 1500, 'hi');
  });
}

async function go() {
  console.log('Async call started...');
  var msg = await message();
  console.log(msg);
}

go();

Upvotes: 1

Jeffrey Devloo
Jeffrey Devloo

Reputation: 1426

The function message does not return the created promise. Normally you'd have an error saying: cannot read property .then of undefined

function message(){    
    var promise = new Promise(function(resolve, reject){
        resolve("hi");
    });
    return promise.then(function(message){  // This will return the initial promise. Due to to the .then, that promise is chained into resolving into the message
       return message;
    })

}

It could be shorted though (in case you do not want the .then in your message function. Just return the promise then:

function message(){    
return new Promise(function(resolve, reject){
    resolve("hi");
});

}

Upvotes: 0

Benoit Esnard
Benoit Esnard

Reputation: 2075

Your message function doesn't return anything.

You could do:

app.get('/message',function(req, res){
    message().then(function (message) {
        res.json(message);
    }
});

function message() {    
    return new Promise(function(resolve, reject){
        resolve("hi");
    });
}

Also, be careful to not use the same names for multiple variables, since it makes the code error-prone due to less readabililty.

Upvotes: 2

Quentin
Quentin

Reputation: 943556

First, you have a local variable named message which masks the module level variable which has the same name and references a function. You need to rename one of them.

Then: You don't have a return statement for the message function, so it returns undefined.

If you want to get the result of the promise back in the callback function you pass to get then you need to:

  1. Return the promise
  2. Call then on it
  3. Use res.json(...); inside the function you pass to then

For example:

app.get('/message',function(req, res){ 
  var my_message = message(); 
  my_message.then(function (data) {
     res.json(data);
 });
});

function message(){    
    var promise = new Promise(function(resolve, reject){
        resolve("hi");
    });
    return promise;
}

Upvotes: 8

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