When working with complex union types in TypeScript, narrowing a value to a specific type is essential for safe and predictable code. However, it is risky to use unsafe narrowing techniques, such as the as type assertion, to force a value into a type that TypeScript cannot guarantee at compile time. The as assertion simply tells the compiler to trust you, bypassing type checks, which can lead to runtime errors if the value is not actually of the asserted type.

Unsafe narrowing can introduce subtle bugs:

• If you use as to treat a value as a more specific type than it really is, you might access properties or call methods that do not exist on the actual value;
• This breaks type safety and undermines the benefits of using TypeScript.

To maintain type safety, you should prefer safer alternatives such as discriminant properties and custom type guard functions.

• Discriminant properties are fields in your types (often called kind or type) that make it easy to distinguish between union members;
• Custom type guard functions are functions that check at runtime whether a value is a particular type, returning a boolean and informing TypeScript of the result.

These techniques allow TypeScript to narrow the type safely, ensuring that your code is robust and less prone to runtime errors.