Suggestion: type narrowing based on user-defined type guards against properties

Search Terms

user-defined type guards, type predicates, type narrowing, object properties

Suggestion

TS can narrow object types in if statements and other conditional control flows by various means. Particularly, if the object's type is a union type distinguishable by a "tag" (like Option<T> in the below example), we can narrow the type by checking the value of the tag. However, if the check for the tag is done by user-defined type guards against the property (but not the object itself), the object's type is not narrowed.

The suggestion is to make it work even if user-defined type guards are applied to object properties

Use Cases

The "tagged union" pattern is widely used in TS codebases. If tags theirselves are so complicated, we want to utilize user-defined type guards to check them.

Examples

type Option<T> = {
  type: "some"
  value: T
} | {
  type: "none"
}

const isSome = (type: "some" | "none"): type is "some" => type === "some"

declare const option: Option<number>

// Good: option is narrowed in the if block
if (option.type === "some") {
  option.value
}

// Bad: option isn't narrowed
if (isSome(option.type)) {
  // Error: Property 'value' does not exist on type 'Option<number>'.
  option.value
}

// Available workaround
const isSomeObject = <T>(option: Option<T>): option is Extract<Option<T>, { type: "some" }> => option.type === "some"

if (isSomeObject(option)) {
  option.value
}

playground

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start with the linked playground and the Option examples to reproduce the difference between direct property checks and user-defined type guards. No implementation file or test is named in the issue, so locate the type-narrowing control-flow logic and its existing tests before defining the change. Done means option.value is accepted after isSome(option.type) while existing narrowing behavior remains unchanged.