Question

8

setTimeout with Promises

rated 0 times [ 11] [ 3] / answers: 1 / hits: 6067 / 4 Years ago, thu, april 9, 2020, 12:00:00

I trying to solve a challenge with the following question:

Implements a function that takes a number as a parameter and after x milliseconds (between an interval of 1 to 100 ms. Use setTimeout and as floor and random functions from the Math library), without a console or with twice the received parameter. Then, call this function 5 times. Ex.:

Without performing any treatment, it is easy to notice that the order of the values shown on the console is random and does not accept the order of calling the functions. Therefore, to resolve this issue, treat or subscribe to setTimeout using callback, Promise and async / waitit.

That's the expected behavior:

let result;



result = double(5, 0); // returns 10

result = double(12, result); // returns 34

result = double(2, result); // returns 38

The goal is to treat the asynchronous behavior of setTimeout with promises, async functions or callbacks.
That's was what i got until now and with no success:



function promisify(number, increase){

    return new Promise(resolve => setTimeout(() => resolve(number * 2 + increase), 100))

}



async function double(number, increase) { 

   const value = await promisify(number, increase); 

   return value;

}



let result; 

result = double(5, 0)

result = double(10, result)

result = double(20, result)



console.log(result)

I want to return a promise with a set timeout that was a random calculate miliseconds until resolve the double of the number + the increase value if it exists.

Even waiting for the promise to result in the async function, it continues to return a pending promise

The result variables have to increase on each calc but they're receiving functions instead of the double result

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answered 4 Years ago andreasn · Accepted Answer

You're actually almost there. All you need is to assign the value of the resolved promise to result, instead of assigning the Promise object directly. This is done by using result = await double(<param1>, <param2>).

However, since JS does not yet support top-level await, you need to wrap your whole result assignment logic in another async function, and then call it as such:

function promisify(number, increase){

    return new Promise(resolve => setTimeout(() => resolve(number * 2 + increase), 100))

}



async function double(number, increase) { 

   const value = await promisify(number, increase); 

   return value;

}



async function run() {

  let result; 

  result = await double(5, 0)

  result = await double(10, result)

  result = await double(20, result)



  console.log(result)

}



run();

However, looking at your code, the double function seems to only serve as a wrapper. Your code can be easily rewritten so that you perform the calculation immediately, but simply wait a little before resolving the promise:

// Simply forces the async operation to wait for a set duration

function wait(duration){

    return new Promise(resolve => setTimeout(resolve, duration));

}



async function double(number, increase) {

   await wait(100);

   return (number * 2) + increase;

}



async function run() {

  let result; 

  result = await double(5, 0)

  result = await double(10, result)

  result = await double(20, result)



  console.log(result)

}



run();

In this way, you can then implement the question's desired randomized settimeout, if you want to, i.e.:

// Waits between [1, 1000] milliseconds

await wait(Math.random() * 1000);