[英]TypeScript error with Promise.reject
Why do I receive the following compilation error with this TypeScript method which I wrote: 为什么我用编写的此TypeScript方法收到以下编译错误:
getFile(path: string): Promise<FileEntry> {
return this.file.resolveLocalFilesystemUrl(path)
.then(entry => {
if (entry.isFile) {
return <FileEntry>entry;
} else {
let err = new FileError(13);
err.message = 'input is not a file';
return Promise.reject(err);
}
});
}
I get the error: 我得到错误:
Argument of type '(entry: Entry) => Promise<never> | FileEntry' is not assignable to parameter of type '(value: Entry) => PromiseLike<never>'. Type 'Promise<never> | FileEntry' is not assignable to type 'PromiseLike<never>'. Type 'FileEntry' is not assignable to type 'PromiseLike<never>'. Property 'then' is missing in type 'FileEntry'.
The definition for resolveLocalFilesystemUrl
is as follows: resolveLocalFilesystemUrl
的定义如下:
resolveLocalFilesystemUrl(fileUrl: string): Promise<Entry>;
FileEntry
is a subclass of Entry
, hence the casted return. FileEntry
是Entry
的子类,因此是强制返回。 However, the error is around Promise.reject
. 但是,该错误与
Promise.reject
。
If I change my reject line to the following, it works: 如果我将拒绝行更改为以下内容,它将起作用:
return Promise.reject<FileEntry>(err);
Do I need to return Promise.reject(...)
and what is the correct implementation? 我是否需要返回
Promise.reject(...)
,正确的实现是什么?
To return a promise you create a new Instance of a promise and return this instance. 要返回承诺,您可以创建一个承诺的新实例并返回该实例。 The constructor takes a function that receives the resolve and reject callbacks you should call, when your logic succeeds or fails.
构造函数采用一个函数,该函数接收逻辑成功或失败时应调用的resolve和拒绝回调。
To let your promise "return" your result, you don't write return result
, but call resolve(result)
. 为了让您的承诺“返回”结果,您不编写
return result
,而是调用resolve(result)
。
getFile(path: string): Promise<FileEntry> {
return new Promise<FileEntry>((resolve, reject) => {
// Your logic here.
// call resolve() on success
// or reject() on failure
});
}
So in your case it should look like the following: 因此,在您的情况下,它应如下所示:
getFile(path: string): Promise<FileEntry> {
return new Promise<FileEntry>((resolve, reject) => {
this.file.resolveLocalFilesystemUrl(path)
.then(entry => {
if (entry.isFile) {
resolve(<FileEntry>entry); // Success
} else {
let err = new FileError(13);
err.message = 'input is not a file';
reject(err); // Error
}
});
});
}
If you want to learn more about promises read for example here: https://developers.google.com/web/fundamentals/primers/promises 如果您想了解有关承诺的更多信息,请阅读以下示例: https : //developers.google.com/web/fundamentals/primers/promises
You shouldn't really return a resolved or rejected promise inside of your promise chain. 您不应该真正在承诺链中返回已解决或被拒绝的承诺。
When you want to indicate an error you should just throw an error: 当您想指出一个错误时,您应该抛出一个错误:
else
let err = new FileError(13);
err.message = 'input is not a file';
throw err;
You can try wrapping the return entry
with a Promise.resolve. 您可以尝试使用Promise.resolve包装
return entry
。 This solved when I had a similar error. 当我遇到类似错误时,此问题解决了。
getFile(path: string): Promise<FileEntry> {
return this.file.resolveLocalFilesystemUrl(path)
.then(entry => {
if (entry.isFile) {
// See this
return Promise.resolve(<FileEntry>entry);
} else {
let err = new FileError(13);
err.message = 'input is not a file';
return Promise.reject(err);
}
});
}
You won't have to write the verbose new Promise((resolve, reject) => {})
because your underlying this.file.resolveLocalFilesystemUrl
already returns a Promise and you can chain on it. 您不必编写冗长的
new Promise((resolve, reject) => {})
因为您的基础this.file.resolveLocalFilesystemUrl
已经返回了Promise,您可以在其上进行链接。 (Which I feel is the correct way.) (我认为这是正确的方法。)
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