函数在等待完成之前返回

Question

I am trying to await for a database query to finish before returning a function, but my approach seems not to work for some reason.

It seems that the function returns before the await returns the resolved Promise .

var returnAfterReading = async () => {
    let myEntityJSONConst = {
        myEntity : {}
    }
    myEntityJSONConst["myEntity"] = await dbQueryPromise();

    return myEntityJSONConst;
}

console.log(JSON.stringify(returnAfterReading())) // {"myEntity":{}}  

What am I doing wrong?

Thank you all in advance.

Answer 1

You're missing await before calling returnAfterReading() function:

Please note the returnAfterReading() function should be called within async context (in async function) or else you need to use promise (eg returnAfterReading().then(data => { /* bla-bla */ }) )

const returnAfterReading = async () => {
    const myEntityJSONConst = {
        myEntity : {}
    }
    myEntityJSONConst["myEntity"] = await dbQueryPromise();

    return myEntityJSONConst;
}

// ... In some other async context
console.log(JSON.stringify(await returnAfterReading()));

Answer 2

It seems that the function returns before the await returns the resolved Promise.

Yes, that is exactly how async functions work. When executing an async function, the moment it hits an await that is awaiting a promise, it immediately returns from the async function. And, the return value is a promise that has not yet been resolved. So, ALL async functions return a promise. The caller of your function will fairly quickly get back a promise (that is not yet resolved). The caller will need to use await (if within a higher level async function) or .then() on that promise to get the eventual resolved value.

Once the promise that is being awaited inside the async function resolves, the function will continue executing. If it hits another await , it will again pause execution. Whatever the eventual return value is from the function's code will then signal the original promise that was returned to be resolved with that value and the caller can then get access to that value via their own await or .then() on the returned promise.

So, assuming that dbQueryPromise(); does actually return a promise that is resolved with its proper value, then you just need to either use await or .then() on the return value from calling returnAfterReading()

const returnAfterReading = async () => {
    let myEntityJSONConst = {
        myEntity : {}
    }
    myEntityJSONConst.myEntity = await dbQueryPromise();

    return myEntityJSONConst;
}

returnAfterReading().then(val => {
     console.log(val);
}).catch(err => {
     console.log(err);
});