typescript의 non-null assertion은 컴파일시에 사라진다

ChatGPT 4o Answer

⚠️ Using TypeScript's Non-Null Assertion Operator (!) Safely

🧠 What Is the Non-Null Assertion Operator?

In TypeScript, the ! operator can be placed after an expression to tell the compiler:

“I know this value is not null or undefined — trust me.”

Example:

const element = document.getElementById('header')!;

This bypasses the compiler’s type checks, assuming element is always present.

🚨 Why It Can Be Dangerous

• Silent runtime failures: If the value actually is null or undefined, the program will throw a runtime error.

• Hidden bugs: This bypasses TypeScript's static safety guarantees, leading to bugs that are hard to trace.

• Overconfidence in external data: It often assumes upstream guarantees (e.g., query results, user input) that might not hold in real-world conditions.

✅ Best Practice: Validate First, Assert Last

Instead of:

const data = fetchDataById(id)!;

Use:

if (!id) {
  throw new Error('Missing id');
}
const data = await fetchDataById(id);
if (!data) {
  throw new Error('Data not found');
}

Or return structured results:

if (!id) return { success: false, reason: 'INVALID_ID' };
const data = await repo.find(id);
if (!data) return { success: false, reason: 'NOT_FOUND' };

🧾 Summary

Rule of thumb: If you're reaching for !, ask yourself: "What if I'm wrong?"

Would you like a Markdown version of this for internal documentation or onboarding?