[英]Javascript promises - catch with/without function
I just started learning JavaScript and promises some hours ago, and I'm starting to get the "flow", but a couple of details are still unclear.几个小时前我刚刚开始学习 JavaScript 和 promises,我开始了解“流程”,但一些细节仍然不清楚。 Let's look at the following example:
让我们看看下面的例子:
function OnRejected(var){
return console.log("This is an error " + var)
}
Promise.reject(2).then(function(a){return a*2})
.then(function(a){return a+5})
.then(function(a){return a*3})
.then(console.log)
.catch(OnRejected)
Result of the above code: This is an error 2上面代码的结果:这是错误2
The example above works just fine.上面的例子工作得很好。 My question is: If I don't call a function and I try to directly call "console.log("this is an error")" inside the catch, why does it fail?
我的问题是:如果我不调用函数,而是尝试在 catch 内直接调用“console.log("this is an error")”,为什么会失败? Like this:
像这样:
Promise.reject(3).then(function(a){return a*2})
.then(function(a){return a+5})
.then(function(a){return a*3})
.then(console.log)
.catch(console.log("This is an error"))
With the result being:结果是:
(node:39515) UnhandledPromiseRejectionWarning: 3 (节点:39515)未处理的承诺拒绝警告:3
This is an error这是一个错误
(node:39515) UnhandledPromiseRejectionWarning: Unhandled promise rejection. (节点:39515)UnhandledPromiseRejectionWarning:未处理的承诺拒绝。 This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch().
这个错误要么是因为在没有 catch 块的情况下抛出了异步函数,要么是因为拒绝了一个没有用 .catch() 处理过的承诺。 (rejection id: 2)
(拒绝编号:2)
(node:39515) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. (节点:39515)[DEP0018] 弃用警告:不推荐使用未处理的承诺拒绝。 In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
将来,未处理的承诺拒绝将以非零退出代码终止 Node.js 进程。
Process finished with exit code 0进程以退出代码 0 结束
More than "promises" I believe my lack of knowledge is about functions in JS, console.log and console.log("whatever").不仅仅是“承诺”,我相信我缺乏关于 JS、console.log 和 console.log(“whatever”) 中的函数的知识。 Any help or advice is really appreciated.
任何帮助或建议都非常感谢。
catch()
and then()
expect to receive a function as argument. catch()
和then()
期望接收一个函数作为参数。 In your example, OnRejected
is a function, while console.log("This is an error")
is not.在您的示例中,
OnRejected
是一个函数,而console.log("This is an error")
不是。
To explain a bit more : console.log
is a function but console.log('something')
is the result of the execution of the function console.log
with the argument 'something'
.再解释一下:
console.log
是一个函数,但console.log('something')
是函数console.log
和参数'something'
的执行结果。
To go back to catch()
and then()
, they will call the method you give it (in your example: OnRejected
) and call it with, as an argument, whatever was return by the previously resolved promise.要返回
catch()
和then()
,他们将调用您提供的方法(在您的示例中: OnRejected
)并使用先前解决的承诺返回的任何内容作为参数调用它。
Example :例子 :
getDataFromDistantServer().then(function (data) => {
console.log(data)
return otherCallToOtherServer()
}).then( function (dataFromOtherServer) => {
console.log(dataFromOtherServer)
})
This would also work because doSomething
is a function:这也可以,因为
doSomething
是一个函数:
var doSomething = function(data) {
console.log(data)
return otherCallToOtherServer()
}
getDataFromDistantServer()
.then(doSomething)
.then( function (dataFromOtherServer) => {
console.log(dataFromOtherServer)
})
Side note : naming convention for your function OnRejected
would dictate to not start the name with a capital letter and call it onRejected
旁注:您的函数
OnRejected
命名约定将规定名称不要以大写字母开头,而是将其onRejected
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