Javascript 更正 node.js 中的异步函数导出

Correct async function export in node.js

提问by Aleksey Kontsevich

I had my custom module with following code:

我的自定义模块带有以下代码：

module.exports.PrintNearestStore = async function PrintNearestStore(session, lat, lon) {
...
}

It worked fine if call the function outside my module, however if I called inside I got error while running:

如果在我的模块外调用该函数它工作正常，但是如果我在内部调用我在运行时出错：

(node:24372) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): ReferenceError: PrintNearestStore is not defined

（节点：24372）UnhandledPromiseRejectionWarning：未处理的承诺拒绝（拒绝ID：1）：ReferenceError：未定义PrintNearestStore

When I changed syntax to:

当我将语法更改为：

module.exports.PrintNearestStore = PrintNearestStore;

var PrintNearestStore = async function(session, lat, lon) {

}

It started to work fine inside module, but fails outside the module - I got error:

它开始在模块内部正常工作，但在模块外部失败 - 我收到错误：

(node:32422) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): TypeError: mymodule.PrintNearestStore is not a function

(node:32422) UnhandledPromiseRejectionWarning: Unhandled promise Rejection (rejection id: 1): TypeError: mymodule.PrintNearestStore is not a function

So I've changed code to:

所以我已经将代码更改为：

module.exports.PrintNearestStore = async function(session, lat, lon) {
    await PrintNearestStore(session, lat, lon);
}

var PrintNearestStore = async function(session, lat, lon) {
...
}

And now it works in all cases: inside and outside. However want to understand semantics and if there is more beautiful and shorter way to write it? How to correctly define and use asyncfunction both: inside and outside (exports) module?

现在它适用于所有情况：内部和外部。但是想了解语义，是否有更漂亮更短的写法呢？如何正确定义和使用异步函数：内部和外部（导出）模块？

回答by Felix Kling

This doesn't really have anything to with async functions specially. If you want to call a function internally andexport it, define it firstand then export it.

这实际上与异步函数没有任何关系。如果要在内部调用函数并导出它，请先定义它，然后再导出它。

async function doStuff() {
  // ...
}
// doStuff is defined inside the module so we can call it wherever we want

// Export it to make it available outside
module.exports.doStuff = doStuff;

---

---

Explanation of the problems with your attempts:

您尝试的问题的解释：

module.exports.PrintNearestStore = async function PrintNearestStore(session, lat, lon) {
...
}

This does not define a function in the module. The function definition is a function expression. The name of a function expression only creates a variable inside the function itself. Simpler example:

这不会在模块中定义函数。函数定义是一个函数表达式。函数表达式的名称仅在函数本身内部创建一个变量。更简单的例子：

var foo = function bar() {
  console.log(typeof bar); // 'function' - works
};
foo();
console.log(typeof foo); // 'function' - works
console.log(typeof bar); // 'undefined' - there is no such variable `bar`

See also Named function expressions demystified. You could of course refer to the function if you'd refer to module.exports.PrintNearestStoreeverywhere.

另见命名函数表达式揭秘。如果您要引用module.exports.PrintNearestStore任何地方，您当然可以引用该函数。

---

---

module.exports.PrintNearestStore = PrintNearestStore;

var PrintNearestStore = async function(session, lat, lon) {

}

This is almostOK. The problem is that the value of PrintNearestStoreis undefinedwhen you assign it to module.exports.PrintNearestStore. The order of execution is:

这几乎没问题。问题是 的值PrintNearestStore是undefined当您将其分配给 时module.exports.PrintNearestStore。执行顺序为：

var PrintNearestStore; // `undefined` by default
// still `undefined`, hence `module.exports.PrintNearestStore` is `undefined`
module.exports.PrintNearestStore = PrintNearestStore;

PrintNearestStore = async function(session, lat, lon) {}
// now has a function as value, but it's too late

Simpler example:

更简单的例子：

var foo = bar;
console.log(foo, bar); // logs `undefined`, `undefined` because `bar` is `undefined`
var bar = 21;
console.log(foo, bar); // logs `undefined`, `21`

If you changed the order it would work as expected.

如果您更改了顺序，它将按预期工作。

---

---

module.exports.PrintNearestStore = async function(session, lat, lon) {
    await PrintNearestStore(session, lat, lon);
}

var PrintNearestStore = async function(session, lat, lon) {
...
}

This works because by the timethe function assigned to module.exports.PrintNearestStoreis executed, PrintNearestStorehas the function as its value.

这工作，因为在时间分配给功能module.exports.PrintNearestStore被执行，PrintNearestStore所具有的功能作为其值。

Simpler example:

更简单的例子：

var foo = function() {
  console.log(bar);
};
foo(); // logs `undefined`
var bar = 21;
foo(); // logs `21`

回答by Grundy

Error with first case: PrintNearestStore- Function expression, so this name not available outside.

第一种情况出错：PrintNearestStore- 函数表达式，因此此名称在外部不可用。

error with second case: using variable, instead Function declaration. In this case, declaration of variable PrintNearestStore are hoisted, so, you can use this name beforeline var PrintNearestStore = ..., but in this case value would be undefined.

第二种情况的错误：使用变量，而不是函数声明。在这种情况下，变量 PrintNearestStore 的声明被提升，因此，您可以在line之前使用此名称var PrintNearestStore = ...，但在这种情况下， value 将为undefined。

So, simplest solution change second variant like this:

因此，最简单的解决方案更改第二个变体，如下所示：

module.exports.PrintNearestStore = PrintNearestStore;

async function PrintNearestStore(session, lat, lon) {

}