Understanding and Solving TypeScript's 'Object is Possibly Undefined' Error Using Optional Chaining and Nullish Coalescing

Introduction

In TypeScript, a common error developers encounter when working with objects that may not be defined is TS2532: Object is possibly 'undefined'. This error occurs because TypeScript is strictly typed to prevent runtime errors by ensuring all variables are properly initialized before use. In this tutorial, we’ll explore how to address this issue using two powerful TypeScript features introduced in version 3.7: Optional Chaining and Nullish Coalescing.

Understanding the Error

When you try to access a property on an object that might be undefined, TypeScript raises an error because it cannot guarantee at compile time that the object is initialized. This safeguard helps prevent runtime errors that could occur if you attempt to access a property of undefined.

Consider this scenario: You have a Firebase Cloud Function that triggers on a Firestore document update and accesses data from the updated document:

export const archiveChat = functions.firestore.document("chats/{chatId}").onUpdate(change => {
  const data = change.after.data(); // Potential source of error if `change` or `change.after` is undefined.
  
  // Additional logic...
});

If either change or change.after could be undefined, TypeScript flags this as an issue, prompting us to use features that can handle such cases gracefully.

Optional Chaining

Optional chaining (?.) allows you to safely access deeply nested properties without having to explicitly check if each reference in the chain is non-null or defined. If any part of the chain evaluates to null or undefined, it short-circuits and returns undefined.

Here’s how you can refactor the code using optional chaining:

const data = change?.after?.data();

In this example, if change is undefined or change.after is undefined, data will be set to undefined without causing an error.

Use Cases for Optional Chaining

Accessing nested object properties where any level might be null or undefined.
Method calls on objects that may not always exist.

Nullish Coalescing Operator

The nullish coalescing operator (??) works alongside optional chaining to provide default values when dealing with null or undefined. This is particularly useful for providing fallbacks in scenarios where an object property might be missing.

Here’s how you can combine it with optional chaining:

const data = change?.after?.data() ?? someOtherData();

If change.after.data() evaluates to undefined, the expression defaults to someOtherData(). This approach ensures that your code has a fallback mechanism, reducing the chances of runtime errors.

Use Cases for Nullish Coalescing

Providing default values when dealing with potentially missing data.
Avoiding unintended falsy value substitutions (e.g., empty strings, zeros).

Implementing in Cloud Functions

Using these features can make cloud function implementations more robust. Here’s a complete example of how you might implement them:

export const archiveChat = functions.firestore.document("chats/{chatId}").onUpdate(change => {
  // Use optional chaining to safely access data
  const data = change?.after?.data();

  if (!data) {
    console.error('No data available');
    return null;
  }

  const maxLen = 100;
  const msgLen = data.messages.length;
  const charLen = JSON.stringify(data).length;

  const batch = db.batch();

  if (charLen >= 10000 || msgLen >= maxLen) {
    // Always delete at least one message
    const deleteCount = Math.max(1, msgLen - maxLen);
    data.messages.splice(0, deleteCount);

    const ref = db.collection("chats").doc(change.after.id);

    batch.set(ref, { ...data }, { merge: true });

    return batch.commit();
  } else {
    return null;
  }
});

In this code, optional chaining and nullish coalescing ensure that the function handles potentially undefined values gracefully.

Best Practices

Use Optional Chaining Judiciously: While optional chaining is powerful, overusing it can mask underlying issues in your data structure. Ensure that the potential for null or undefined values is expected.
Default Values with Caution: When using nullish coalescing to provide default values, ensure they are appropriate and won’t introduce logic errors.
TypeScript Version Compatibility: Remember that optional chaining and nullish coalescing require TypeScript 3.7 or later. Ensure your project configuration supports these features.

Conclusion

By leveraging optional chaining and nullish coalescing, you can write safer and more concise TypeScript code. These features help manage undefined values elegantly, reducing runtime errors and improving the robustness of your applications. Whether working with complex data structures or integrating with third-party services like Firebase Cloud Functions, these tools are invaluable for modern TypeScript development.