How to set a JS variable depending on the duration of an asynchronous call

Question

I have this:

this.toggleWaiting()
this.results = await this.query(term)
this.toggleWaiting()

First a loading spinner gets triggered. Then the query runs. And when the query function ended the loading spinner gets closed.

But what if I want to just show the loading spinner, when the query takes maybe more then 0.5 seconds?

Is there a simple way to do this?

Answer 1

A way to achieve this is to pass the this.query(term) promise to a function which will handle triggering the toggleWaiting only when the query takes longer than the specified amount of time (using the timeout).

For example, the below takes a promise, a function (waitingFn) which will be called with the isWaiting status as well as a timeout which you can use to specify how long you want to wait before you show the loading spinner. Finally, when the promise has been fulfilled, we return the result:

async function handleWaiting(promise, waitingFn, timeout) {
  let loadingStarted = false;
  let timeoutInstance = null;

  const timeoutPromise = new Promise((res) => {
    timeoutInstance = setTimeout(() => {
      loadingStarted = true;
      waitingFn(true);
    }, timeout);
    return res();
  });

  function onFinished() {
    clearTimeout(timeoutInstance);

    if (loadingStarted) {
      waitingFn(false);
    }
  }

  try {
    const [result] = await Promise.all([promise, timeoutPromise]);
    onFinished();
    return result;
  } catch (ex) {
    onFinished();
    throw ex;
  }
}

You can call the handleWaiting function like so:

const result = await handleWaiting(this.query(term), (isWaiting) => this.toggleWaiting(), 500);

As @FZs and @Bergi have pointed out (thank you both), the below is an antipattern due to using the promise constructor:

function handleWaiting(promise, waitingFn, timeout) {
  return new Promise((res, rej) => {
     let loadingStarted = false;
   
     const timeoutInstance = setTimeout(() => {
       loadingStarted = true; 
       waitingFn(true);
     }, timeout);

     function onFinished() {
       if (loadingStarted) {
         waitingFn(false);
       }
       clearTimeout(timeoutInstance);
     }
     
     return promise
       .then((result) => {
         onFinished();
         res(result);
       })
       .catch((ex) => {
         onFinished();
         rej(ex);
       });
  });
}