类中的异步/等待：意外令牌`this`

Question

I am experimenting with async/await, I can't understand why this line :

resolvedValue = await this.tryToSolve()

gives me this error :

Unexpected token this

 class Test { constructor() { this.method = 0 this.checkLink() } async checkLink() { return new Promise((resolve, reject) => { let resolvedValue for (let i = 0; i < 3; i++) { this.method = i resolvedValue = await this.tryToSolve() if (resolvedValue) break } console.log(`Method ${this.method} did the trick.`); resolve(resolvedValue) }) } tryToSolve() { return new Promise((resolve, reject) => { // Resolves if this.method==1 console.log(`Trying to solve with method ${this.method}...`); setTimeout(() => { resolve(!!this.method ? `http://www${this.method}.someurl.com` : false) }, 1000) }) } } const test = new Test() 

Does anyone know the correct syntax to store the result of an async method in a variable?

Thanks in advance.

Answer 1

To keep things simple, it happens because when you create a Promise, in its' constructor you pass an arrow function, which contains await call. You must always put async keyword before the declaration of a function, that contains await .

So, instead of doing this

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

        let resolvedValue

        for (let i = 0; i < 3; i++) {
            this.method = i
            resolvedValue = await this.tryToSolve()
            if (resolvedValue) break
        }
        console.log(`Method ${this.method} did the trick.`);
        resolve(resolvedValue)
    })
}

Do it like this

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

        let resolvedValue

        for (let i = 0; i < 3; i++) {
            this.method = i
            resolvedValue = await this.tryToSolve()
            if (resolvedValue) break
        }
        console.log(`Method ${this.method} did the trick.`);
        resolve(resolvedValue)
    })
}

More info: https://ponyfoo.com/articles/understanding-javascript-async-await#using-async-await

Answer 2

Drop the new Promise around the await ! You want only

async checkLink() {
    let resolvedValue
    for (let i = 0; i < 3; i++) {
        this.method = i
        resolvedValue = await this.tryToSolve()
        if (resolvedValue) break
    }
    console.log(`Method ${this.method} did the trick.`);
    return resolvedValue;
}

or much simpler

async checkLink() {
    for (let i = 0; i < 3; i++) {
        const value = await this.tryToSolve()
        if (value) {
            console.log(`Method ${i} did the trick.`);
            return value;
        }
    }
}