for loop, inside async setter doesn't wait all Promise. #3898

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1. aduh95 commented on Jun 19, 2022

   

   aduh95

   on Jun 19, 2022

   Last edited by aduh95

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   The promise your setter returns in ignored by the engine, it will treat it as a truthy boolean value and that's all. So when you write await (_instance.attributes = attrs);, _instance.attributes = attrs will evaluate to the value of attrs, not the Promise your setter returns.

   If you add a setTimeout(() => console.log(foo.attributes), 300);, you'll see that all the properties are there eventually.

   Trying to set property asynchronously is dangerous, you can get into race conditions very easily if more than one async branch tries to write data concurrently, I wouldn't recommend it.

   If async setters is really what you want, here are some (not so good) examples

   You'd either need to expose some property that returns a promise that resolves when the instance is ready:

   class Foo {
     static async build(attrs = {}) {
       const _instance = new this();
       _instance.attributes = attrs;
       await _instance.areAttributesReady;
       return _instance;
     }
   
     #props = {};
     #propsPromise = Promise.resolve();
   
     get areAttributesReady() {
       return this.#propsPromise;
     }
   
     set attributes(attrs) {
       this.#propsPromise = (async (previousPromise) => {
         await previousPromise;
         await Promise.all(Object.entries(attrs).map(async ([key, value]) => {
           this.#props[key] = await value;
         }));
         this.#propsPromise = Promise.resolve();
       })(this.#propsPromise);
     }
   
     get attributes() {
       return this.#props;
     }
   }
   
   async function main() {
     const attrs = { q: 'q', w: 'w', e: 'e', r: 'r', a: 'a' };
     const foo = await Foo.build(attrs);
     console.log(foo.attributes); // incomplete.
   }
   
   main().catch(console.log);

   Or make an async method:

   class Foo {
     static async build(attrs = {}) {
       const _instance = new this();
       await _instance.setAttributes(attrs);
       return _instance;
     }
   
     #props = {};
   
     async setAttributes(attrs) {
         await Promise.all(Object.entries(attrs).map(async ([key, value]) => {
           this.#props[key] = await value;
         }));
     }
   
     get attributes() {
       return this.#props;
     }
   }
   
   async function main() {
     const attrs = { q: 'q', w: 'w', e: 'e', r: 'r', a: 'a' };
     const foo = await Foo.build(attrs);
     console.log(foo.attributes); // incomplete.
   }
   
   main().catch(console.log);

   👍React with 👍1Reacted by Taihwan Hah
2. aduh95

   transferred this issue fromnodejs/nodeon Jun 19, 2022
3. duecorda commented on Jun 19, 2022

   

   duecorda

   on Jun 19, 2022

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   Thank you for your kind answer.
   May I need change my structure.
4. duecorda

   closed this as completedon Jun 19, 2022