TypeScript(error:2532): object may be undefined despite null or undefined check

Question

TypeScript(error:2532): object may be undefined despite null or undefined check

Currently, I am developing a Discord-bot using TypeScript which includes a command to retrieve information from an airport. To do this, users only need to provide the 4-character ICAO code that identifies the specific airport. However, due to potential user errors in inputting the ICAO code, I have implemented a method that retrieves the airport object from a JSON file. Here is a simplified representation of how the method looks:

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

In my command file/class, I utilize this method to fetch the airport based on the provided ICAO code. Subsequently, I check if the returned value is undefined and handle it accordingly by displaying an error message. This portion of the code appears as follows:

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

*additional logic*

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

On rare occasions, I encounter an error in two distinct files/classes where I use this particular method together with the defined checks. The error suggests that the 'airport' object may be undefined. Strangely, when accessing some other property of 'airport' a few lines below, no error is indicated.

While I could resolve this issue temporarily by utilizing as Airport when retrieving or reassigning the airport, I aim to identify a more robust solution rather than circumventing TypeScript rules. Can anyone provide insights on how to effectively address this perplexing problem?

edit:

A reference picture showing the area where the code worked correctly can be viewed here: https://i.stack.imgur.com/tTZyJ.png

typescript discord undefined

Answer 1

Answer №1

The issue lies in your use of let instead of const. When using let, the variable can be reassigned, potentially changing its type throughout the code. Here's an example to illustrate this concept:

// Making it async for demonstration purposes; adapt as needed
async function doStuff() {
  const icao = 'some icao';
  let airport: Airport | undefined = getAirport(icao);

  if (airport === undefined) {
    return;
  }

  // Exploring different scenarios based on lexical scope
  airport.name;

  // Function within current lexical scope
  function someHelperFunction() {
    airport.name;
  }

  // Promise resolution values don't retain original type
  Promise.resolve().then(() => {
    airport.name
  });

  // Works with simple awaits
  const anything = await Promise.resolve('something');
  airport.name;

  // Creating a constant of known type for consistency
  const airportForSure: Airport = airport;
  function someOtherHelperFunction() {
    airportForSure.name;
  }

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

  airport = getAirport(icao); // Value change requires reevaluation
  
  airport.name; // Will not work post-reassignment
}

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

// Placeholder for actual function implementation
declare function getAirport(icao: string): Airport | undefined;

Link to TypeScript playground

Answer 2

The issue lies in your use of let instead of const. When using let, the variable can be reassigned, potentially changing its type throughout the code. Here's an example to illustrate this concept:

// Making it async for demonstration purposes; adapt as needed
async function doStuff() {
  const icao = 'some icao';
  let airport: Airport | undefined = getAirport(icao);

  if (airport === undefined) {
    return;
  }

  // Exploring different scenarios based on lexical scope
  airport.name;

  // Function within current lexical scope
  function someHelperFunction() {
    airport.name;
  }

  // Promise resolution values don't retain original type
  Promise.resolve().then(() => {
    airport.name
  });

  // Works with simple awaits
  const anything = await Promise.resolve('something');
  airport.name;

  // Creating a constant of known type for consistency
  const airportForSure: Airport = airport;
  function someOtherHelperFunction() {
    airportForSure.name;
  }

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

  airport = getAirport(icao); // Value change requires reevaluation
  
  airport.name; // Will not work post-reassignment
}

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

// Placeholder for actual function implementation
declare function getAirport(icao: string): Airport | undefined;

Link to TypeScript playground

TypeScript(error:2532): object may be undefined despite null or undefined check

Answer №1

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