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Please complete the following 9 questions
1. Multiple.catch
var p = new Promise((resolve, reject) => { reject(Error('The Fails!')) }) p.catch(error => console.log(error.message)) p.catch(error => console.log(error.message))
What will be the output of the above code? Please choose the correct answer:
UnhandledPromiseRejectionWarningAnalysis:
We use the constructor method to create a Promise and pass therejectcallback The error is triggered immediately.
Then.catchworks similar to DOM's.addEventListener(event, callback)or Event Emitter's.on(event, callback), wherecan add multiple callbacks.Each is called with the same parameters.
2. Multiple.catch##
var p = new Promise((resolve, reject) => { return Promise.reject(Error('The Fails!')) }) p.catch(error => console.log(error.message)) p.catch(error => console.log(error.message))The output of the above code will be What? Please choose the correct answer:
Analysis:
resolve()orreject()callback. The Promise constructor does not use your return value, so no other Promises created byPromise.reject()will actually be received.
.catchafterPromise.reject(), the answer isUnhandledPromiseRejectionWarning.
3. Link.thenand.catch
var p = new Promise((resolve, reject) => { reject(Error('The Fails!')) }) .catch(error => console.log(error)) .then(error => console.log(error))What will be the output of the above code? Please choose the correct answer:
Parsing
.thenand.catch, it's helpful to think of them as a series of steps. Each.thenreceives as its argument the value returned by the previous.then. However, if your "step" encounters an error, any subsequent.then"steps" will be skipped until.catchis encountered. If you want to override an error, all you have to do is return a non-error value. Can be accessed via any subsequent.then.
Tip:console.log()always returnsundefined.
4. Link.catch##
var p = new Promise((resolve, reject) => { reject(Error('The Fails!')) }) .catch(error => console.log(error.message)) .catch(error => console.log(error.message))What will be the output of the above code? ? Please choose the correct answer:
[x] Print the error message once
When chaining
.catch, each only processes the previous.thenor`.catchError raised in "step". In this example, the first.catchreturnsconsole.log, which can only be achieved by adding.then() after the two.catchCome visit.
5. Multiple.catch##
new Promise((resolve, reject) => { resolve('Success!') }) .then(() => { throw Error('Oh noes!') }) .catch(error => { return "actually, that worked" }) .catch(error => console.log(error.message))The output of the above code will be What? Please choose the correct answer:
[ ] Print the message once
Tip:
.catchcan be done simply by returning a regular value to ignore (or override) errors.This trick only works when subsequent.then
.then
Promise.resolve('Success!') .then(data => { return data.toUpperCase() }) .then(data => { console.log(data) })The output of the above code will What will it be? Please choose the correct answer:
[ ] print "Success!" and "SUCCESS!"
Tips: .then passes the data in sequence, starting fromreturn value
to the next.then(value => /* handle value */).In order to pass the value to the next.then
returnis the key.
.then
Promise.resolve('Success!') .then(data => { return data.toUpperCase() }) .then(data => { console.log(data) return data }) .then(console.log)The output of the above code will What will it be? Please choose the correct answer:
解析：
有两个console.log调用将被调用。
8..then之间的流程
Promise.resolve('Success!') .then(data => { data.toUpperCase() }) .then(data => { console.log(data) })
以上代码的输出将会是什么？请选择正确的答案：
undefined解析：
提示：.then依次传递数据，从返回值到下一个.then(value => /* handle value */)。
为了将值传递给下一个.then，return是关键。
9..then和.catch之间的流程
Promise.resolve('Success!') .then(() => { throw Error('Oh noes!') }) .catch(error => { return 'actually, that worked' }) .then(data => { throw Error('The fails!') }) .catch(error => console.log(error.message))
以上代码的输出将会是什么？请选择正确的答案：
解析：
本文转载自：https://segmentfault.com/a/1190000021255822
英文原文地址： https://danlevy.net/javascript-promises-quiz/
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