诺言解决有什么问题? [英] What is wrong with promise resolving?
问题描述
有什么想法吗?为什么节点说文件名未定义"?谢谢. 合同，政策和发票功能无需数据即可解决，只需resolve().
Any ideas? Why does node say 'filename is undefined'? Thanks. Contract, policy ans invoice functions resolve with no data, just resolve().
var dc = function(data) {
return new Promise(function(resolve, reject) {
var filename = 'Test';
var contract = function() { ... }
var policy = function() { ... }
var invoice = function() { ... }
contract().then(invoice().then(policy().then(function() {
console.log(filename); // Test
resolve(filename); // UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): ReferenceError: filename is not defined
})))
})
}
推荐答案
首先，您不能编写:
contract().then(invoice() ... )
(如果invoice()函数返回另一个函数作为then处理程序，则可以使用)
(that would work if the invoice() function returned another function to act as a then handler)
您必须写:
contract().then(function (value) { invoice() ... })
或者:
contract().then(value => invoice() ... )
或者，如果一个功能应处理另一功能的结果，则可能是这样:
Or maybe this if one function should handle the result of other function:
contract().then(invoice).then(policy).then(function (result) { ... });
必须将作为参数传递给then的是函数，而不是调用函数的结果(在您的示例中这可能是一个承诺).
What you have to pass as an argument to then is a function, not a result of calling a function (which is probably a promise in your example).
我不知道这是否是您的方法中唯一的问题，但这当然是问题之一.当然可以，但是可能不如您期望的那样.
I don't know if that's the only problem with your approach but it is certainly one of the problems. Of course it may work but probably not how you expect.
如果您使用的是ES2017 async/await，则自v7.0起在Node中可用，而不是:
If you use ES2017 async/await that's available in Node since v7.0 then instead of:
contract().then(invoice).then(policy).then((result) => { ... });
您可以使用:
let a = await contract();
let b = await invoice(a);
let c = await policy(b);
// here your `result` is in `c`
甚至这个:
let result = await policy(await invoice(await contract()));
请注意，您只能在用async关键字声明的函数中使用它.这适用于7版以上的Node.对于较旧版本的Node，您可以使用基于生成器的协程，使用语法稍有不同的类似内容，也可以使用Babel来编译代码(如果您愿意的话)做.
Note that you can only use it in functions declared with the async keyword. This works on Node since version 7. For older versions of Node you can use a similar thing with a slightly different syntax using generator-based coroutines, or you can use Babel to transpile your code if that's what you prefer of if that what you already do.
这是一个相当新的功能，但是有关堆栈溢出的问题很多.参见:
This is quite a new feature but there are a lot of questions on Stack Overflow about it. See:
- 尝试/捕获具有async/await的块
- 是否以阻止程序语言的方式进行异步?
- 尝试/捕获具有async/await的块
- 在异步外部使用await
- 在Babel 6中使用acyns/await与Babel
- 何时进行异步方法抛出以及如何捕获它们?
- 使用诺言node.js创建和比较两个数组
- 函数将从javascript post/get中返回null
- try/catch blocks with async/await
- Do async in a blocking program language way?
- try/catch blocks with async/await
- Use await outside async
- Using acyns/await in Node 6 with Babel
- When do async methods throw and how do you catch them?
- using promises in node.js to create and compare two arrays
- Keeping Promise Chains Readable
- function will return null from javascript post/get
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