Variables are not being set inside the .then() part of a Promise

Question

I'm pretty new to JavaScript and node.js and so being thrown into Promises is pretty daunting when it is required for my bot to function.

var storedUserID;
ROBLOX.getIdFromUsername(accountArg).then(function(userID) {
  message.reply("your id is " + userID);
  storedUserID = userID;
});
message.reply(storedUserID);

This is essentially what I have written out, it creates a variable called 'storedUserID' which I would like to update later. I am trying to update it in the Promise but it does not seem to work.

In the code, there is message.reply("your id is " + userID); which works as expected. It will print out to a user 'your id is [NUMBER]' so I know userID is not null.

However, when I run message.reply(storedUserID); outside of the Promise, nothing is printed as the variable is not saved. I am not sure why.

Any help would be appreciated as this is going towards my work in college! Thank you!

The `then` is run asynchronously. By the time you log that callback hasn't executed yet. — Ingo Bürk, Nov 04 '17 at 16:04
@IngoBürk See OP at _"which I would like to update later."_ How does linked Question and answers demonstrate how to change the value of a `Promise`? — guest271314, Nov 04 '17 at 16:20

guest271314 · Accepted Answer · 2017-11-04T16:15:58.010

12

return the value at function passed to .then()

var storedUserID = ROBLOX.getIdFromUsername(accountArg)
                   .then(function(userID) {
                     return userID;
                   });

you can then use or change the Promise value when necessary

storedUserID = storedUserID.then(id) {
  return /* change value of Promise `storedUserID` here */
});

access and pass the value to message.reply() within .then()

storedUserID.then(function(id) {
  message.reply(id);
});

or

var storedUserID = ROBLOX.getIdFromUsername(accountArg);
// later in code
storedUserID.then(function(id) {
  message.reply(id);
});

edited Nov 04 '17 at 16:15

answered Nov 04 '17 at 16:07

guest271314

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What's the point of mapping the promise to itself? – Ingo Bürk Nov 04 '17 at 16:09
@IngoBürk Do you mean the second example at updated post? – guest271314 Nov 04 '17 at 16:11
Yes. But my question stands: why is that the second alternative? What's the point of the first one? – Ingo Bürk Nov 04 '17 at 16:11
1

@IngoBürk To demonstrate that the value of a `Promise` can be changed at `.then()` by returning a different value, see OP at _"which I would like to update later."_ – guest271314 Nov 04 '17 at 16:13
Ah, I see what's going on.. thanks! Still kinda confused with this whole asynchronous thing. :] – WelpNathan Nov 04 '17 at 17:49
@WelpNathan See [You're Missing the Point of Promises](https://gist.github.com/domenic/3889970) – guest271314 Nov 04 '17 at 17:50

score 0 · Answer 2 · answered Nov 04 '17 at 16:09

Because the getIdFromUsername is asynchronous, the final line where you message.reply the stored ID is run before the ID is stored.

Asynchronous functions like getIdFromUsername don't stop the following lines of code from running. If you want it to happen after the ID is returned it needs to go inside the then callback.

Variables are not being set inside the .then() part of a Promise

2 Answers2