Does a conditional wait for member variable update in JS?

Question

I just have a basic question about order of execution in JS code.

deleteAttachement({ cdId, paId: recordId }).then(response => {
                if (response) {
                    this.existingFilesList = this.existingFilesList.filter(file => (file.contentVersionId != cvId));
                }
                if(this.existingFilesList.length == 0) {
                    this.statusIcon = this.missingIcon;
                }
            }).catch(err => {
                console.log(err);
            });

Im trying to change the Icon based on the amount of records contained in the member variable "this.existingFilesList" (Its an array).

Im just wondering if in the code above, the second if conditional waits untill this.existingFilesList.filter() function finishes and the member variable is updated, or the filter() function just runs asynchronously in the background and as soon as it is called, the if conditional is called practically at the same time without waiting for the this.existingFilesList update.

It seems like it works and wait for it, but Im not sure if its due to the short amount of records.

Why would it run asynchronously? `filter` is a synchronous method. — Bergi, Apr 24 '23 at 22:52
Check out this Q/A about the [JavaScript event loop](https://stackoverflow.com/questions/21607692/understanding-the-event-loop) :). If you still have questions I can type something out. — Joe Lissner, Apr 25 '23 at 14:22

score 0 · Answer 1 · answered Apr 24 '23 at 21:53

this.existingFilesList.filter(file => (file.contentVersionId != cvId));

Every function call in JavaScript always returns a value immediately. (Unless an exception is thrown, but I hope that is obvious.)

The value might be undefined or a Promise, but a value will always be returned.

If the value is a Promise and your were expecting an array or some other value, then you have a bug in your code. You need to wait for the Promise to resolve using either .then(callback) or await.

So, the short answer to your question depends on what you mean by "this.existingFilesList.filter() function finishes".

I'm assuming that existingFilesList is a regular array, in which case filter executes synchronously, so the simple answer is "Yes... the second if conditional waits until this.existingFilesList.filter() function finishes."

If, however, existingFilesList is some custom object or class and filter() returns a Promise, then you have a bug because this.existingFilesList will be set to the returned Promise and "the second if conditional" will not wait for the Promise to be resolved. It will keep on trucking and this.existingFilesList.length == 0 will evaluate as false because undefined == 0 is false.

Some free advice:

If possible, I would highly recommend using the async and await keywords. It is much easier to read and understand what is going on and leads to fewer bugs. (If, for example, your existingFilesList is something other than an array and filter returns a Promise, then you could add an await in front of it without needing to start a new .then() chain)

async function deleteStuff() {
  try {
    const response = await deleteAttachement({ cdId, paId: recordId });

    if (response) {
      this.existingFilesList = this.existingFilesList.filter(file => (file.contentVersionId != cvId));
    }
    if(this.existingFilesList.length == 0) {
      this.statusIcon = this.missingIcon;
    }
  } catch (err) {
    console.log(err);
  }
}

Does a conditional wait for member variable update in JS?

1 Answers1