I'm trying to translate a code from Promise returning one to async/await. My example code has 2 classes.

Example code

First one, TestAsync, functions are async. This class prints 2 first, then 1. Which means they run parallel and doesn't wait for the first one to finish.

class TestAsync {
    private async f1(n: any) {
        setTimeout(() => {
            console.log("Async 1: "   n);
            return n;
        }, 1500);
    }

    private async f2(n: any) {
        setTimeout(() => {
            console.log("Async 2: "   n);
            return n;
        }, 1000);
    }

    public async f() {
        await this.f1(1)
        await this.f2(2);
    }
}

In the second one, TestPromise, functions return Promises without async fucntions. This class prints 1 first, then 2. That's what I expected (waiting for the first one to finish) also the TestAsync should do.

class TestPromise {
    private f1(n: any) {
        return new Promise((resolve, reject) => {
            setTimeout(() => {
                resolve(n);
                console.log("Promise 1: "   n);
            }, 1500);
        });
    }

    private f2(n: any) {
        return new Promise((resolve, reject) => {
            setTimeout(() => {
                resolve(n);
                console.log("Promise 2: "   n);
            }, 1000);
        });
    }

    public async f() {
        await this.f1(1);
        await this.f2(2);
    }
}

How can I make TestAsync function to behave like TestPromise?

CodePudding user response：

async/await are mostly syntactic sugar around promises. So when dealing with APIs that are not marked as async or returning a promise - such as setTimeout in your example -, they'll still need to be converted into promises.

In your TestAsync example, the functions don't return anything so await doesn't have anything to wait for.

You might want to run callback-based functions through promisify or use the new timers promise API in Node.js: https://nodejs.org/api/timers.html#timers-promises-api Then they can be used in conjunction with await.

CodePudding user response：

the error is that you are not waiting for setTimeout to finish, you need to wrap it in a promise and await for it.

function wait(t) {
    return new Promise(resolve => setTimeout(resolve, t));
}
class TestAsync {
    async f1(n) {
        await wait(1500);
        console.log("Async 1: "   n);
        return n;
    };

    async f2(n) {
        await wait(1000);
        console.log("Async 2: "   n);
        return n;
    };

    async f() {
        await this.f1(1)
        await this.f2(2);
    };
}

new TestAsync().f()