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 return 一个将在 2 秒后解析的承诺。以下代码不应编辑，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

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()

解释如下：

使用 setTimeout()，这是一种内置方法，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 promise 总是会解决

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

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

延迟后的承诺

Promise after delay

javascript

closures

javascript-objects

es6-promise