TypeScript Error Handling

TypeScript Error Handling: Unknown Errors & Custom Error Classes

In strict TypeScript, errors caught inside try...catch blocks are typed as unknown (or any). Proper error handling requires narrowing caught errors before accessing properties like error.message.

Error Handling Architecture Flow

flowchart TD
    A["try { ... } catch (err)"] --> B["err is typed as unknown"]
    B --> C{"err instanceof Error?"}
    C -- "Yes" --> D["Access err.message & err.stack cleanly"]
    C -- "No" --> E["Handle raw string / unknown throw payload"]

Practical Code Example

// Custom Domain Error Class
export class DatabaseConnectionError extends Error {
  public readonly code: string = "DB_CONN_FAILED";

  constructor(message: string, public readonly host: string) {
    super(message);
    this.name = "DatabaseConnectionError";
    Object.setPrototypeOf(this, DatabaseConnectionError.prototype);
  }
}

function connectToDatabase(host: string): void {
  if (host === "invalid_host") {
    throw new DatabaseConnectionError("Failed to resolve database host", host);
  }
  console.log(`Successfully connected to ${host}`);
}

// Type-Safe Error Handling
try {
  connectToDatabase("invalid_host");
} catch (err: unknown) {
  if (err instanceof DatabaseConnectionError) {
    console.error(`[${err.code}] ${err.message} (Host: ${err.host})`);
  } else if (err instanceof Error) {
    console.error(`Standard Error: ${err.message}`);
  } else {
    console.error("Unknown throw payload:", err);
  }
}

Best Practices & Gotchas

• Always Reset Prototype Chain in Custom Errors: Call Object.setPrototypeOf(this, CustomError.prototype) inside custom Error constructors to ensure instanceof works properly across ES targets.
• Never Throw Raw Strings: Always throw new Error(...) instead of throwing raw primitives like throw "something failed".
• Use Result Types for Expected Operations: Consider returning { ok: true, value: T } | { ok: false, error: E } instead of relying on throw for non-exceptional business logic.

Self-Check Challenge

What is the type of err inside catch (err) when useUnknownInCatchVariables or strict is enabled? (Answer: unknown)