TypeScript(err:2532)：object可能是未定义的清空/未定义检查

Question

我一直在用TypeScript编写一个不和谐的机器人，我添加了一个命令从机场检索信息。您唯一需要提供的是机场的民航组织代码(识别机场的4个字符代码)。现在:很明显，用户可能会搞错国际民航组织的代码，可能会给出无效的代码。这就是为什么我有一个从JSON检索机场对象的方法，它看起来有点像这样(不是一个精确的副本，只是为了让您了解它)：

public getAirport(icao: string): Airport | undefined {
    return arrayOfAllAirports.find(ap => ap.icao === icao);
}

在我的命令文件/类中，我使用此方法明显地使用给定的民航组织代码检索机场。然后我检查它的返回值是否是未定义的，如果是的话，我是return *some error*，就是这样。看起来就像这样：

let airportMgr = new AirportManager();
    
let airport = airportMgr.getAirport(icao);
if (airport === undefined) {
    return *invalid icao error*;
}

*blablabla*

let exampleString = `ICAO: ${airport.icao} | Name: ${airport.name}`;
                                    ^error here             ^error here

但是在我使用上述方法的两个文件/类以及上述检查中，我会得到一个更低的错误，对象可能是未定义的。现在出现了更令人困惑的地方:低几行。我又进入了“机场”的一些财产，却什么也没说。

现在我知道当我检索机场或重新分配它时我可以使用as Airport，但是我想要一个正确的解决方案，而不是以愚蠢的方式欺骗类型记录。你们中有谁对如何处理这个问题有什么想法吗？

编辑：

这是它的工作地点的图片：https://i.stack.imgur.com/tTZyJ.png

Answer 1

问题是您使用的是let，而不是const。let可以重新分配，这意味着每次可能发生更改时，它的类型都会被扩展到原来的类型。就像这样：

// I made it async to demonstrate something below, for your case it is not important
async function doStuff() {
  // Some icao code
  const icao = 'some icao';
  let airport: Airport | undefined = getAirport(icao);

  // Let's start by checking whether the airport is undefined
  if (airport === undefined) {
    return;
  }

  // And now let's break down all the scenarios that work
  // and then all the ones that don't
  //
  // The important concept here is called lexical scope,
  // an "environment" in which variables and constants are defined

  // 1. current lexical scope - the scope of this function
  //
  // This should work since the value of airport has just been checked
  // and has not had an opportunity to be changed
  airport.name;

  // 2. "child" lexical scope - scope created by a function defined within
  // the current lexical scope
  // 
  // This should not work since you might be calling this function
  // some later time, when the value of airport would have possibly changed
  function someHelperFunction() {
    airport.name;
  }

  // The same applies to promise resolution values
  Promise.resolve().then(() => {
    // This should not work
    airport.name
  });

  // It works with simple awaits
  const anything = await Promise.resolve('something');
  // This will work
  airport.name;

  // A TRICK (!!!)
  //
  // You can at this point create a const of type Airport (because now you are sure
  // it is an airport) and use that in your functions.
  //
  // Since it is defined as const, it cannot be changed 
  // and it will stay an airport like forever and ever
  const airportForSure: Airport = airport;
  function someOtherHelperFunction() {
    // WORKS!!!
    airportForSure.name;
  }

  Promise.resolve().then(() => {
    // Yesss
    airportForSure.name
  });

  // Then when you change the value of airport, you need to check for undefined again
  airport = getAirport(icao);

  // This will not work
  airport.name;
}

type Airport {
  icao: string;
  name: string;
}

// Just a placeholder for a function
declare function getAirport(icao: string): Airport | undefined;

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