为什么我无法通过同步等待从异步等待中获得价值？

Question

I am trying to get mongodb instance synchronously. I know this is not recommended but I just experiment and wonder why this doesn't work. this.db is still undefined after 10 seconds of waiting when normally asynchronous code gets it in less than 500 milliseconds.

Repository.js:

var mongodb = require('mongodb');
var config = require('../config/config');

var mongoConfig = config.mongodb;
var mongoClient = mongodb.MongoClient;

class Repository {
    constructor() {
        (async () => {
            this.db = await mongoClient.connect(mongoConfig.host);
        })();
    }

    _getDb(t) {
        t = t || 500;
        if(!this.db && t < 10000) {
            sleep(t);
            t += 500;
            this._getDb(t);
        } else {
            return this.db;
        }
    }

    collection(collectionName) {
        return this._getDb().collection(collectionName);
    }
}

function sleep(ms) {
    console.log('sleeping for ' + ms + ' ms');
    var t = new Date().getTime();
    while (t + ms >= new Date().getTime()) {}
}

module.exports = Repository;

app.js:

require('../babelize');

var Repository = require('../lib/Repository');
var collection = new Repository().collection('products');

Answer 1

Javascript is an event-based architecture. All code is initiated via an event from the event queue and the next event is pulled from the event queue ONLY when the code from the previous event has finished executing. This means that your main Javascript code is single threaded.

As such, when you fire an async operation, it starts up an operation and when that operation finishes, it puts an event in the event queue. That event (which will trigger the callback for the async operation) will not run until the code from the previous event finishes running and returns back to the system.

So, now to your code. You start running some code which launches an async operation. Then, you loop forever and never return back to the system. Because of that, the next event in the event queue from the completion of your async operation can NEVER run.

So, in a nutshell, you cannot spin in a loop waiting for an async operation to complete. That's incompatible with the event driven scheme that Javascript uses. You never return back to the event system to let the async completion callback ever run. So, you just have a deadlock or infinite loop.

Instead, you need to code for an async response by returning a promise or by passing in a callback that is called sometime later. And your code needs to finish executing and then let the callback get called sometime in the future. No spinning in loops in Javascript waiting for something else to run.

You can see the async coding options here: How do I return the response from an asynchronous call?