Type narrow with switch(obj.constructor)

[pauldraper]

Suggestion

🔍 Search Terms

constructor property, type narrowing

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

I would like the .constructor property to narrow the type in a switch statement.

📃 Motivating Example

Today, the following fails type checking:

class Success<T> { constructor(readonly value: T) {} }
class Failure { constructor(error: any) {} }
type Result<T> = Success<T> | Failure;

const result: Result<string> = new Success("example");

switch (result.constructor) {
  case Success:
    console.log(`Success: ${result.value}`);
    break;
  case Failure:
    console.log(`Failure: ${result.error}`);
    break;
}

💻 Use Cases

Ad-hoc polymorphism.

Many people think this already works.

Request (2018) redirected to a (completed, but different) issue: #2214 (comment)

Blog post (2020) claiming this feature exists: https://log.havrlant.cz/typescript-switch-instanceof/ (Sadly, the author's demonstration was flawed.)

if can work instead, but gives up the performance advantages of switch for a lot of branches.

[fatcerberus]

Duplicate of #16035.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.