Using nested async await function calls

May 25, 2022

I have a very simple code I’m trying to debug. I have an async function:

async function updateResult(event){

  let result = db.fetchResult(event.ProcessId);
  return result;

}

And I’m calling this from another simple async function:

exports.processEvent = async (event) => {
  try {

    let promises = [];


    const controls = await db.fetchControls(event.EventCode);
    
    if (controls) {     
      
       promises = controls.map((control) => {
         console.log(control);
         control.map(x => this.updateResult(event));
        
       });
           
     }

    return Promise.allSettled(promises);
  }
  catch (err) {
    console.error(err);
  }
};

The problem is that in exports.processEvent, the content of db.fetchControls(event.EventCode) is executed to completion (a few logics and a db getItem call).

But the call to db.fetchResult(event.ProcessId) via this.updateResult(x.Id, x.Version, event) does NOT complete executing the fetchResult tasks (a few other logics a db GET call).

I feel like fetchResult returns prematurely. What am I doing wrong?

>Solution :

.map() is NOT promise-aware. It does not wait for any of the promises in your callback to complete. So, in this piece of code:

control.map(x => this.updateResult(x.Id, x.Version, event));

It just runs through the entire array ignoring all the promises that are returned and thus NOTHING waits for those promises.

You can fix that by returning those promises to the higher level .map() which you are then awaiting with Promise.all():

  promises = controls.map((control) => {
     console.log(control);
     return Promise.all(control.map(x => this.updateResult(x.Id, x.Version, event)));
  });