延后承诺

如何解决延后承诺

我有这个问题，我似乎不知道该如何解决？编写一个承诺的函数，它接受一个值。此函数将返回一个承诺，该承诺将在2秒后解决。以下代码不应编辑，而应使用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

解决方法

const sleep = async (time) => {
    return new Promise(resolve => setTimeout(resolve,time * 1000))
}



const sayHello = async () => {
    await sleep(2)
    console.log('Hello there')
}


sayHello()

以下是说明：

使用setTimeout()是一种内置方法，它在指定的毫秒数后调用函数或对表达式求值。 setTimeout()接受2个参数，第一个是回调函数，第二个是毫秒数。 1秒= 1000ms，所以2秒= 2000ms，依此类推

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)) 

除非您100％确信诺言将始终得以解决，否则应始终使用.catch

示例：

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

了解有关承诺https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise

的更多信息