Passing values from one promise to the next

Question

A bit new to javascript. Been dealing with promises, however ran into a problem i dont know how to approach.

How can i pass a value into the next promise resolve?

Here's my code

bot.on('ask.add_account_password', (msg) => {
    let username = users[msg.from.id].username;
    let password = msg.text;
    var accounts = require('./inc/account.js');
    accounts.login_account(username,password).then(function(data){
        var account = data.params;
        console.log(account);
        return accounts.store(msg.from.id,username,password,account.followerCount);
    }).then(function(data){
        let caption = "Your account "+account.username+"("+account.fullName+")has been added\n";
        return bot.sendPhoto(msg.from.id, account.picture, {caption:caption});
    })
    .catch(function(error){
        console.log(error);
        add_account(msg,error.name);
    });
});

On the line where i create the caption variable, i'm trying to access the account object created in the block before it(var account = data.params) but i get a reference error saying its not defined. Now i can easily bypass this by just sending the entire object into the accounts.store function and have it resolve the object when done, but that just seems like a dirty workaround to a bigger problem. Is there a cleaner way to do this?

guest271314 · Answer 1 · 2017-07-30T15:23:04.710

1

account is undefined at second .then(), use data to reference the Promise value accounts.store returned from previous .then()

.then(function(data) {
  // `data` : `accounts.store` returned from previous `.then()`
  let caption = "Your account " + data.username 
                + "(" + data.fullName + ")has been added\n";
  return bot.sendPhoto(msg.from.id, data.picture, {caption:caption});
})

edited Jul 30 '17 at 15:23

answered Jul 30 '17 at 15:15

guest271314

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What makes you think that `store` returns a (promise for) the `account`? – Bergi Jul 30 '17 at 17:41
@Bergi _"What makes you think that store returns a (promise for) the account"_ Not sure what you mean? The gist of the Answer is that if OP needs to access a value from the previous `.then()` the returned value from previous `.then()` is the parameter at subsequent `.then()`. OP needs to make sure they are returning the correct value that they are expecting at subsequent `.then()` – guest271314 Jul 30 '17 at 17:49
Yes, but they `return accounts.store(…)` not `account` so this doesn't work that easily. – Bergi Jul 30 '17 at 17:51
@Bergi It was not immediately clear from OP exactly which object OP was expecting to return from previous `.then()`, nor exactly which object properties were expected at chained `.then()`. We had not gotten that far as to precisely what OP was returning or expecting at chained `.then()`. Not sure how your canonical can assist in that particular matter? Though OP will be able to review different possible patterns of accessing `Promise` values. – guest271314 Jul 30 '17 at 17:57

score 1 · Answer 2 · answered Jul 30 '17 at 15:17

1

You can create variable (and set it in promise) in main function or you can return this as result of first promise instead of result from function store

answered Jul 30 '17 at 15:17

hsd

452
5
12

score 0 · Answer 3 · answered Jul 30 '17 at 15:18

0

You can pass it in an array and use destructuring in the arguments to unpack the array.

accounts.login_account(username,password).then(function(data){
    var account = data.params;
    console.log(account);
    return [account, accounts.store(msg.from.id,username,password,account.followerCount)];
}).then(function([account, data]){
    let caption = "Your account "+account.username+"("+account.fullName+")has been added\n";
    return bot.sendPhoto(msg.from.id, account.picture, {caption:caption});
})

You don't actually appear to be using data, so you could just return account after calling store

answered Jul 30 '17 at 15:18

the8472

40,999
5
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The first propose solution is dangerous in context of promises. When function `store` return promise then chain of promises will be broken. – hsd Jul 30 '17 at 17:40
You would need to use `Promise.all` to make sure all promises in the array get awaited – Bergi Jul 30 '17 at 17:42

Passing values from one promise to the next

3 Answers3