Promises期约
挑战1
问题:
让我们从复习异步函数开始吧!使用setTimeout,在1000ms后打印字符串“Hello!”。
题解:
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// Challenge 1 functionsayHello() { // 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:
创建一个promise。延迟1000ms后在resolve里面传入值“Resolved!”,使用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:
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// Challenge 2 var promise = newPromise(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:
创建另外一个promise。这次不使用setTimeout,在reject里面传入值“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:
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// Challenge 3 promise = newPromise(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。现在让我们来证明它们确实如此!创建一个promise,在resolve里面传入值“Promise has been resolved!”,然后去掉挑战4底部的注释。运行后我们看到的打印顺序是怎样的?“Promise has been resolved!”先还是”I’m not the promise!”先?为什么?
题解:
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// Challenge 4 promise = newPromise(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:
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// Challenge 5 functiondelay(){ returnnewPromise(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:
在这个挑战中我们会使用.then链式调用promises期约。创建两个变量: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的方式调用firstPromise,这会返回secondPromise,然后在它的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:
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// Challenge 6 // // ADD CODE BELOW var secondPromise = Promise.resolve('Second!') var firstPromise = newPromise(function (resolve, reject) { resolve(secondPromise) }) firstPromise.then().then(res =>console.log(res)) |
挑战7
问题:
我们有一个会从数据库拉取数据的API,它接收一个下标参数然后返回一个promise。你的挑战是使用promise.all发起3次API调用,然后在调用都结束后返回对应的数据。
题解:
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// 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); returnnewPromise((resolve, reject) => { if (i >= 0 && i < fakePeople.length) { setTimeout(() => resolve(fakePeople[i]), returnTime); } else { reject({ message: "index out of range" }); } }); }; functiongetAllData() { // CODE GOES HERE returnPromise.all([fakeAPICall(0), fakeAPICall(1), fakeAPICall(2)]) } getAllData().then(res =>console.log(res)) |