类别中的Async/Await:意外的标记`this` [英] Async/Await in a Class : unexpected token `this`

问题描述

我正在尝试异步/等待，我不明白为什么这行:

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

resolvedValue = await this.tryToSolve()

给我这个错误:

此意外令牌

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()

有人知道将异步方法的结果存储在变量中的正确语法吗?

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

谢谢.

推荐答案

为简单起见，它的发生是因为创建Promise时，在其构造函数中传递了一个箭头函数，该函数包含await调用.您必须始终将async关键字放在包含await的函数的声明之前.

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.

所以，不要这样做

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)
    })
}

像这样

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)
    })
}

更多信息: https://ponyfoo.com/article/understanding-javascript-async-await#using-async-await

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