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Please complete the following 9 questions
1. Multiple<span style="font-size: 18px;">.catch</span>
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:
UnhandledPromiseRejectionWarning
Analysis:
We use the constructor method to create a Promise and pass the reject
callback The error is triggered immediately.
Then .catch
works similar to DOM's .addEventListener(event, callback)
or Event Emitter's .on(event, callback)
, where can 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:
When using the Promise constructor, you must callresolve() or
reject() callback. The Promise constructor does not use your return value, so no other Promises created by
Promise.reject() will actually be received.
.catch after
Promise.reject(), the answer is
UnhandledPromiseRejectionWarning.
3. Link .then and
.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
When chaining.then and
.catch, it's helpful to think of them as a series of steps. Each
.then receives 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
.catch is 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 returns
undefined.
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:
When chaining
.catch, each only processes the previous .then
or `.catch
Error raised in "step". In this example, the first .catch
returns console.log
, which can only be achieved by adding .then() after the two
.catch
Come 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:
.catch can be done simply by returning a regular value to ignore (or override) errors. This trick only works when subsequent
.then
.thenPromise.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 from return value
to the next .then(value => /* handle value */).
In order to pass the value to the next
.then
return is the key.
.thenPromise.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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