Adding a promise to an array using Javascript

Question

Adding a promise to an array using Javascript

I am facing an issue while attempting to create an array of promises and then calling them using Promise.all.

The problem lies in correctly pushing the functions into the array. It seems like they are getting executed instead of being inserted and waiting for Promise.all() to handle them.

function findSpecialAbility(players, gameId, message) {
  return new Promise(function(resolve, reject) {
    let playersWithSpecials = _.reject(players, function(p) {
      return p.role === 'alphaWolf' ||
        p.role === 'betaWolf' ||
        p.role === 'villager' ||
        p.role === 'alchemist' ||
        p.targetId === 0 ||
        p.abilityUsed === true;
    });
    if (playersWithSpecials.length === 0) {
      resolve();
    } else {
      let specialsToUse = [];
      for (let i = 0, j = playersWithSpecials.length; i < j; i++) {
        specialsToUse.push(useSpecialAbility(playersWithSpecials[i], gameId, message, players));
      }
      //Promise.all(specialsToUse).then(r = > console.log(r));
    }
  });
}


// Using promise below because some of the role will have database updates.
function useSpecialAbility(playerData, gameId, message, players) {
  return new Promise(function(resolve, reject) {
    if (playerData.role === 'seer') {
      let getTargetData = _.find(players, {
        id: playerData.targetId
      });
      message.guild.members.get(playerData.id).send(`Your target is a ${getTargetData.role}!`);
      resolve('foo');
    }
  });
}

javascript arrays function es6-promise

Answer 1

Answer №1

There appears to be a misconception that they are being invoked instead of inserted and are waiting for Promise.all()

It seems like the intention is to have the code within the promises execute concurrently when Promise.all is called.

If that is the desired outcome, then enclosing the code in the promises may not be the best approach.

Instead, what needs to be done is wrapping the code intended for later execution in a simple function. These functions can then be added to an array and executed in sequence. Looking at the provided code, it's likely that promises might not even be necessary.

Refer to the example below:

// This function returns another function `runnable` which can be synchronously called later to obtain the result
function runLater (index) {
  return function runnable() {
    console.log(`this code will run later from ${index}`);
    return `from ${index}`;
  }
}

console.log('This runs now');

const tasks = [];

for (let i = 0; i < 3; i += 1) {
  tasks.push(runLater(i));
}

console.log('This runs second');


// Use Array.prototype.map to map the array of functions to their results by invoking each one.
const valuesOfTasks = tasks.map(task => task());
console.log({valuesOfTasks});

console.log('This runs last');

Promises are only needed when handling asynchronous control flow. Delaying execution can be achieved synchronously by encapsulating a section of code within a function. When you wish to run that code, simply invoke the function.

Edit:

I need all useSpecialAbility calls to finish before proceeding with my code. Some involve database read/write operations, hence the use of promises.

In this scenario, you would indeed have to utilize Promise.all. However, the promises must still be wrapped in functions without any arguments if simultaneous execution is desired.

The block of promise code actually executes synchronously (as opposed to running the block on .then or Promise.all invocation), so if postponing the execution of promises is still desired, simply enclose them in functions.

Consider the following example:

function waitThenSay(milliseconds, message) {
  return new Promise(resolve => {
    console.log(`waiting to say: "${message}"...`)
    setTimeout(() => {
      // Log to console for immediate effect
      console.log(message.toUpperCase() + '!');
      // Resolve with the message as well
      resolve(message);
    }, milliseconds);
  });
}

console.log('Runs first');

// Tasks is an array of functions returning promises
const tasks = [];
for (let i = 1; i <= 3; i += 1) {
  tasks.push(() => waitThenSay(i * 2000, `hello from task ${i}`));
}

console.log('Runs second');

// Execute the tasks by mapping each function to its invocation
const arrayOfPromises = tasks.map(task => task())

// Call Promise.all on that array
Promise.all(arrayOfPromises).then(result => {
  console.log({result});
});

Hope this clarifies things!

Answer 2

There appears to be a misconception that they are being invoked instead of inserted and are waiting for Promise.all()

It seems like the intention is to have the code within the promises execute concurrently when Promise.all is called.

If that is the desired outcome, then enclosing the code in the promises may not be the best approach.

Instead, what needs to be done is wrapping the code intended for later execution in a simple function. These functions can then be added to an array and executed in sequence. Looking at the provided code, it's likely that promises might not even be necessary.

Refer to the example below:

// This function returns another function `runnable` which can be synchronously called later to obtain the result
function runLater (index) {
  return function runnable() {
    console.log(`this code will run later from ${index}`);
    return `from ${index}`;
  }
}

console.log('This runs now');

const tasks = [];

for (let i = 0; i < 3; i += 1) {
  tasks.push(runLater(i));
}

console.log('This runs second');


// Use Array.prototype.map to map the array of functions to their results by invoking each one.
const valuesOfTasks = tasks.map(task => task());
console.log({valuesOfTasks});

console.log('This runs last');

Promises are only needed when handling asynchronous control flow. Delaying execution can be achieved synchronously by encapsulating a section of code within a function. When you wish to run that code, simply invoke the function.

Edit:

I need all useSpecialAbility calls to finish before proceeding with my code. Some involve database read/write operations, hence the use of promises.

In this scenario, you would indeed have to utilize Promise.all. However, the promises must still be wrapped in functions without any arguments if simultaneous execution is desired.

The block of promise code actually executes synchronously (as opposed to running the block on .then or Promise.all invocation), so if postponing the execution of promises is still desired, simply enclose them in functions.

Consider the following example:

function waitThenSay(milliseconds, message) {
  return new Promise(resolve => {
    console.log(`waiting to say: "${message}"...`)
    setTimeout(() => {
      // Log to console for immediate effect
      console.log(message.toUpperCase() + '!');
      // Resolve with the message as well
      resolve(message);
    }, milliseconds);
  });
}

console.log('Runs first');

// Tasks is an array of functions returning promises
const tasks = [];
for (let i = 1; i <= 3; i += 1) {
  tasks.push(() => waitThenSay(i * 2000, `hello from task ${i}`));
}

console.log('Runs second');

// Execute the tasks by mapping each function to its invocation
const arrayOfPromises = tasks.map(task => task())

// Call Promise.all on that array
Promise.all(arrayOfPromises).then(result => {
  console.log({result});
});

Hope this clarifies things!

Answer 3

Answer №2

Here is a simple async/await method for adding promises to an array and using Promise.all on them afterward.

async (req, res, next) => {
    try{
        const promises = [];
        
        //Conditionally insert promises into the array

        if(req.user.hobby === "Hockey")
            promises.push(() => Hobby.find({name: "Hockey"}));

        else
            promises.push(() => Hobby.find({}));

        if(req.user.country === "Canada")
            promises.push(() => User.find({country: "Canada"}));
        
        //Execute the promises using Promise.all()            
                
        const docs = await Promise.all(promises.map(promise => promise()));
    
        if(docs.length === 0){
            var err = new Error('No documents found');
            err.status = 404;
            return next(err);
        }        

        if(docs[1])
            console.log(docs[1]); //array of User docs with country field value set to Canada
    
        console.log(docs[0]); //array of all hobbies or only hobbies with the name field set to Hockey

        res.json(docs);
    }
    catch(err){
        return next(err);
    }
}

Answer 4

Here is a simple async/await method for adding promises to an array and using Promise.all on them afterward.

async (req, res, next) => {
    try{
        const promises = [];
        
        //Conditionally insert promises into the array

        if(req.user.hobby === "Hockey")
            promises.push(() => Hobby.find({name: "Hockey"}));

        else
            promises.push(() => Hobby.find({}));

        if(req.user.country === "Canada")
            promises.push(() => User.find({country: "Canada"}));
        
        //Execute the promises using Promise.all()            
                
        const docs = await Promise.all(promises.map(promise => promise()));
    
        if(docs.length === 0){
            var err = new Error('No documents found');
            err.status = 404;
            return next(err);
        }        

        if(docs[1])
            console.log(docs[1]); //array of User docs with country field value set to Canada
    
        console.log(docs[0]); //array of all hobbies or only hobbies with the name field set to Hockey

        res.json(docs);
    }
    catch(err){
        return next(err);
    }
}

Adding a promise to an array using Javascript

Answer №1

Answer №2

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