需要NodeJS中的异步功能

Question

I am trying to get my head around async/await in NodeJS.

I have a function in a file as follows:

const getAccessToken = async () => {
  return new Promise((resolve, reject) => {

    const oauthOptions = {
      method: 'POST',
      url: oauthUrl,
      headers: {
        'Authorization': 'Basic ' + oauthToken
      },
      form: {
        grant_type: 'client_credentials'
      }
    };

    request(oauthOptions)
      .then((err, httpResponse, body) => {
        if (err) {
          return reject('ERROR : ' + err);
        }
        return resolve(body.access_token);
      })
      .catch((e) => {
        reject('getAccessToken ERROR : ' + e);
      });
  });
};

module.exports = getAccessToken;

This file is saved as twitter.js in a lib folder

In my index.js file I have the following:

const getAccessToken = require('./lib/twitter');

let accessToken;

try {
  accessToken = await getAccessToken();
} catch (e) {
  return console.log(e);
}

console.log(accessToken);

I get an error trying to run this code saying:

>   accessKey = await getAccessToken();
>                     ^^^^^^^^^^^^^^
> 
> SyntaxError: Unexpected identifier
>     at createScript (vm.js:74:10)
>     at Object.runInThisContext (vm.js:116:10)
>     at Module._compile (module.js:533:28)
>     at Object.Module._extensions..js (module.js:580:10)
>     at Module.load (module.js:503:32)
>     at tryModuleLoad (module.js:466:12)
>     at Function.Module._load (module.js:458:3)
>     at Function.Module.runMain (module.js:605:10)
>     at startup (bootstrap_node.js:158:16)
>     at bootstrap_node.js:575:3

Can I not await the required function as it is marked async ?

Answer 1

Your code is already correct. Without changing anything in twitter.js you have two options to use it:

1. Functions marked with async always return promises. Therefore you can simply do:

    const getAccessToken = require('./lib/twitter'); getAccessToken().then(accessToken => { console.log(accessToken); }) 

2. All functions that return promises can be awaited upon but you cannot use await outside of an async marked functions. So you can also do:

    const getAccessToken = require('./lib/twitter'); (async function () { var accessToken = await getAccessToken(); console.log(accessToken); })(); 

The async/await keywords does not change how async functions behave. You still cannot wait on async functions synchronously, you can only do that inside an asynchronous function. Async/await is just a syntax sugar on top of promises.

Answer 2

You are on the right track, however await can only be used inside an async function. So you have your structure backwards, and can be easily solved by changing a few things. However I suggest you overlook your code and make some structural changes. But here is a working version of your code:

const getAccessToken = () => {
  return new Promise((resolve, reject) => {

    const oauthOptions = {
      method: 'POST',
      url: oauthUrl,
      headers: {
        'Authorization': 'Basic ' + oauthToken
      },
      form: {
        grant_type: 'client_credentials'
      }
    };

    request(oauthOptions)
      .then((err, httpResponse, body) => {
        if (err) {
          return reject('ERROR : ' + err);
        }
        return resolve(body.access_token);
      })
      .catch((e) => {
        reject('getAccessToken ERROR : ' + e);
      });
  });
};

module.exports = getAccessToken;

And then:

const getAccessToken = require('./lib/twitter');

(async function() {
  try {
    let accessToken = await getAccessToken();
    console.log(accessToken);
  } catch (e) {
    return console.log(e);
  }
})()

This is a simple fix to your code to illustrate that you've got it backwards, and some structural changes are in order.