关于javascript:JavaScript专项算法题6Promises

@冒泡的马树

题库原地址:http://csbin.io/promises

Promises期约

挑战1

问题:

让咱们从温习异步函数开始吧!应用setTimeout,在1000ms后打印字符串“Hello!”。

题解:

// Challenge 1

function sayHello() {
  // ADD CODE HERE
  setTimeout(() => console.log('Hello!'), 1000)
}

// Uncomment the line below when ready
sayHello(); // should log "Hello" after 1000ms

挑战2

问题:

创立一个promise。提早1000ms后在resolve外面传入值“Resolved!”,应用setTimeout。在promise被调用resolve后,打印它的内容,借助传入console.log到.then中来实现。

题解:

// Challenge 2
var promise = new Promise(function (resolve, reject) {
  // ADD CODE HERE
  setTimeout(() => resolve('Resolved'), 1000)
});

// Should print out "Resolved!"
// ADD CODE HERE
promise.then(res => console.log(res))

挑战3

问题:

创立另外一个promise。这次不应用setTimeout,在reject外面传入值“Rejected!”。在promise被调用reject之后,打印它的内容,借助传入console.log到.catch来实现。

题解:

// Challenge 3

promise = new Promise(function(resolve, reject) {
  // ADD CODE HERE
  reject('Rejected')
})

// Should print out "Reject!"
// ADD CODE HERE
promise.catch(res => console.log(res))

挑战4

问题:

Promises期约是异步的。当初让咱们来证实它们的确如此!创立一个promise,在resolve外面传入值“Promise has been resolved!”,而后去掉挑战4底部的正文。运行后咱们看到的打印程序是怎么的?“Promise has been resolved!”先还是”I’m not the promise!”先?为什么?

题解:

// Challenge 4

promise = new Promise(function (resolve, reject) {
  // ADD CODE HERE
  resolve()
});

// Uncomment the lines below when ready
promise.then(() => console.log('Promise has been resolved!'));
console.log("I'm not the promise!");

挑战5

问题:

编写delay函数,用于返回一个promise。此返回promise应该返回一个在1000ms后调用resolve的setTimeout。

题解:

// Challenge 5
function delay(){
    return new Promise(function (resolve, reject) {
        return setTimeout(resolve, 1000)
  })
}

// Uncomment the code below to test
// This code should log "Hello" after 1000ms
delay().then(sayHello);

挑战6

问题:

在这个挑战中咱们会应用.then链式调用promises期约。创立两个变量:firstPromise和secondPromise,让secondPromise成为一个在resolve中传值“Second!”的promise,firstPromise则在resolve中传入secondPromise。用.then的形式调用firstPromise,这会返回secondPromise,而后在它的resolve执行之后,打印对应promise的内容,借助传入console.log到.then来实现。

题解:

// Challenge 6
//
// ADD CODE BELOW
var secondPromise = Promise.resolve('Second!')
var firstPromise = new Promise(function (resolve, reject) {
  resolve(secondPromise)
})
firstPromise.then().then(res => console.log(res))

挑战7

问题:

咱们有一个会从数据库拉取数据的API,它接管一个下标参数而后返回一个promise。你的挑战是应用promise.all发动3次API调用,而后在调用都完结后返回对应的数据。

题解:


// Challenge 7
const fakePeople = [
  { name: 'Rudolph', hasPets: false, currentTemp: 98.6 },
  { name: 'Zebulon', hasPets: true, currentTemp: 22.6 },
  { name: 'Harold', hasPets: true, currentTemp: 98.3 },
]

const fakeAPICall = (i) => {
  const returnTime = Math.floor(Math.random() * 1000);
  return new Promise((resolve, reject) => {
    if (i >= 0 && i < fakePeople.length) {
      setTimeout(() => resolve(fakePeople[i]), returnTime);
    } else {
      reject({ message: "index out of range" });
    }
  });
};

function getAllData() {
  // CODE GOES HERE
  return Promise.all([fakeAPICall(0), fakeAPICall(1), fakeAPICall(2)])
}

getAllData().then(res => console.log(res))

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