延迟后的承诺

Question

我有这个问题，我似乎不知道如何解决？写一个函数，承诺，接受一个值。此函数将返回一个承诺，该承诺将在 2 秒后解决。下面的代码不应该被编辑并且应该使用 promise 对象。 function promised (val) {         }        // UNCOMMENT THESE TO TEST YOUR WORK!     const createPromise = promised('wait for it...');     createPromise.then((val) => console.log(val));     // will log "wait for it..." to the console after 2 seconds

Answer 1

const sleep = async (time) => {

    return new Promise(resolve => setTimeout(resolve, time * 1000))

}

const sayHello = async () => {

    await sleep(2)

    console.log('Hello there')

}

sayHello()

这是解释：

使用setTimeout()which 是一种内置方法，可在指定的毫秒数后调用函数或计算表达式。 setTimeout()有两个参数，第一个是回调函数，第二个是毫秒数。1 秒 = 1000 毫秒所以 2 秒 = 2000 毫秒等等

function promised (val) {

    // Create a new promise and resolve val after 2 seconds

    return new Promise(resolve => setTimeout(() => resolve(val), 2000)) //2000ms => 2 seconds

}

const createPromise = promised('wait for it...') // Pass in your message to promised function

createPromise

    .then(val => console.log(val))

    // Catch errors if any you don't need it here since we are always resolving the promise i just included it here so you know it's exist 

    .catch(err => console.log(err)) 

你应该总是使用 .catch 除非你 100% 确定承诺总是会解决

例子：

function greeting(name) {

    return new Promise((resolve, reject) => setTimeout(() => {

        

        try {

            

            if (name.length <= 2) {

                throw new Error('Name must be more than two characters')

            }

        } catch (msg) {

            reject(msg)

        } 

        resolve(`Hello ${name}`)

        

        

    }, 2000))

}

greeting('ab')

    .then(res => console.log(res))

    .catch(err => console.log(err)) // Error: Name must be more than two characters

greeting('John')

    .then(res => console.log(res)) // Hello John

    .catch(err => console.log(err))

使用异步，等待：

const greetSomeone = async (name) => {

    try {

        // Note that you can't use await outside an async function

        const msg = await greeting(name) 

        console.log(msg)

    } catch (err) {

        console.log(err)

    }

}

greetSomeone('ac') // Error: Name must be more than two characters

greetSomeone('Michael') // Hello Michael