Async function returning promise, instead of value

问题

I'm trying to understand how async/await works in conjunction together with promises.

Code

async function latestTime() {
  const bl = await web3.eth.getBlock('latest');
  console.log(bl.timestamp); // Returns a primitive
  console.log(typeof bl.timestamp.then == 'function'); //Returns false - not a promise
  return bl.timestamp;
}
const time = latestTime(); // Promise { <pending> }

Issue

As far as I understand, await should be blocking and in the code above it seemingly blocks returning an object bl with the primitive timestamp. Then, my function returns the primitive value, however the time variable is set to a pending promise instead of that primitive. What am I missing?

回答1:

Async prefix is a kind of wrapper for Promises.

async function latestTime() {
    const bl = await web3.eth.getBlock('latest');
    console.log(bl.timestamp); // Returns a primitive
    console.log(typeof bl.timestamp.then == 'function'); //Returns false - not a promise
    return bl.timestamp;
}

Is the same as

function latestTime() {
    return new Promise(function(resolve,success){
        const bl = web3.eth.getBlock('latest');
        bl.then(function(result){
            console.log(result.timestamp); // Returns a primitive
            console.log(typeof result.timestamp.then == 'function'); //Returns false - not a promise
            resolve(result.timestamp)
        })
}

回答2:

async function will return Promise anyway. Return value will be `Promise, so in your case it will be:

async function latestTime(): Promise<some primitive> {
  const bl = await web3.eth.getBlock('latest');
  return bl.timestamp;
}

So, further you can use it function like:

const time = await latestTime();

But for achieving general view about async/await feature it will be better to read documentation.

来源：https://stackoverflow.com/questions/51338277/async-function-returning-promise-instead-of-value