异步ES2017构造函数

Question

What is the most up to date method to ensure that some asynchronous code completes in a class constructor before that class is subsequently used?

Specifically, how would an API client class retrieve an access token before allowing more method calls, as shown below?

class API_Client {

    constructor(...) {

        # Below should 'block' other method calls until token is assigned
        this.login().then(res => {
            this.token = res.data.token;
        });

    }

    async login() {
        return makeRequest(...) # <-- Promise which returns access token data
    }
}

const client = new API_Client(...);
client.someAuthOnlyMethod() # <-- Should only happen after the `login` method completes.

I found older answers , yet couldn't quite understand how to solve the problem posed in the first comment left on the linked answer.

Answer 1

The most up-to-date method is still not to put any asynchronous stuff in the constructor . In your specific case, that's

class API_Client {
    constructor(token) {
        this.token = token;
    }
    static async createLoggedIn(…) {
        const res = await makeRequest(...) # <-- Promise which returns access token data
        return new this(res.data.token);
    }
}

const client = await API_Client.createLoggedIn(…);
client.someAuthOnlyMethod()

Answer 2

You could store the token as a promise:

class API_Client {

    constructor(...) {

        # Below should 'block' other method calls until token is assigned
        this.token = this.login()
          .then(res => res.data.token)

    }

    async someAuthOnlyMethod() {
      let token = await this.token;
      //...continue
    }

    async login() {
        return makeRequest(...) # <-- Promise which returns access token data
    }
}

const client = new API_Client(...);
client.someAuthOnlyMethod() # <-- Should only happen after the `login` method completes.

Answer 3

You shouldn't be calling any asynchronous code from a constructor to begin with. In the case above, your makeRequest function would worry about the login token.

There is also no real value in a class in this case. You should just export a series of functions to make the API calls.