How to make async wait for result

Question

I have a class like this (simplified):

export class Foo {

    constructor(el, menuItems ) {
        this.id = this.el.dataset.id;
        this.menuItems = menuItems;

        this.fetchResult = async function (uri) {

            fetch(uri, {
                method: 'get'
            })
                .then(response => response.json())
                .then(data => {
                    return data.saved === true;
                })
                .catch(function (err) {
                    console.log('Failed ', err);
                    return false;
                });
        }

    }

    doAction(action) {

        const uri  = 'foo/'+ action +'?id=' + this.id;

        const map = {
            'close': async () => {
                const success = await this.fetchResult(action);
                console.log(success); // says undefined? 
            }
            /* some other actions */
        }
        
        return map[action];
    }

    attachDialogs() {
        this.menuItems.addEventListener('click', (element) => {
            this.doAction( element.dataset.action )();
        });
    }    
}

For some reason the const success = await this.fetchResult(action); does not wait for the result. It immediately proceeds, resulting in and 'undefined' value.

In the background the fetch is being done. How can I make it wait for the result?

try to make your doAction function async and then call it in the eventListener with await ? your "doAction" function return a function whose async but the function itself isn't async — Toto NaBendo, Aug 20 '21 at 14:05
You never return anything from `fetchResult`. You probably also mean to pass `uri` to `fetchResult`, not `action`. [Many dupes](https://html.duckduckgo.com/html?q=async%20function%20returns%20undefined%20site%3Astackoverflow.com). — ggorlen, Aug 20 '21 at 14:05

score 1 · Answer 1 · answered Aug 20 '21 at 15:19

I believe the issue is that you are not resolving the response from your request, try returning a promise and resolving only when you have your response (or error)

this.fetchResult = function(uri) {
  return new Promise((resolve, reject) => {
    fetch(uri, {
      method: 'get'
    })
    .then(response => response.ok
      ? response.json()
      : reject(new Error(response.statusText)))
    .then(result => resolve(result))
  }
}


const map = {
  'close': async () => {
     const success = await this.fetchResult(action);
   }
}

Remove the async from fetchResult there is no reason for it. You're awaiting the result of the Promise in fetchResult, you only need the async keyword if fetch result is written in async await as well:

await res = fetch(url)

score 0 · Answer 2 · answered Aug 20 '21 at 20:45

Thanks @Toto Nabendo and @ggorlen!

Working solution:

export class Foo {

    constructor(el, menuItems ) {
        this.id = this.el.dataset.id;
        this.menuItems = menuItems;

        this.fetchResult = async function (uri) {

            return fetch(uri, {
                method: 'get'
            })
                .then(response => response.json())
                .then(data => {
                    return data.saved === true;
                })
                .catch(function (err) {
                    console.log('Failed ', err);
                    return false;
                });
        }

    }

    async doAction(action) {

        const uri  = 'foo/'+ action +'?id=' + this.id;

        const map = {
            'close': async () => {
                const success = await this.fetchResult(action);
                console.log(success);  
            }
            /* some other actions */
        }
        
        return map[action];
    }

    attachDialogs() {
        this.menuItems.addEventListener('click', (element) => {
            this.doAction( element.dataset.action )();
        });
    }    
}

How to make async wait for result

2 Answers2