Accept de-structured elements in type predicates

Search Terms

• type predicate
• reference binding pattern
• type predicate cannot reference
• destructured

Suggestion

The possibility to use destructured parameters in type predicates.

Use Cases

Destructuring is heavily used in functional/reactive programming, notably with rxjs where various contextual properties tend to be passed between each operator.

Having the ability to succinctly test for types would make the code more readable, e.g.:

type Example = {
  a: number;
  b: string | undefined;
};

const example: Example = {
  a: 42,
  b: 'hello';
};

of(example).pipe(
  guard(({ b }): b is string => b !== undefined, 'b cannot be undefined'),
  tap({ b }) => { /* b is now a string rather than a string | undefined })
);

Right now the alternative is

of(example).pipe(
  guard((x): x is Omit<typeof x, 'b'> & { b: string } => x.b !== undefined, 'b cannot be undefined'),
  tap({ b }) => { /* b is now a string rather than a string | undefined })
);

Or, without a predicate

of(example).pipe(
  map(x => {
    if (x.b === undefined) {
      throw new Error();
    }

    return x;
  }),
  tap({ b }) => { /* b is now a string rather than a string | undefined })
);

Examples

function assertSomething(
  { property }: T
): property is AssertionHere {
  return true;
}

This would roughly translate to something like:

function assertSomething(
  obj: T
): obj is Omit<T, 'property'> & { property: AssertionHere } {
  return true;
}

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.

No files, tests, or compiler entry points are identified in the issue. Start by reading the type-predicate and destructured-parameter examples, then trace how TypeScript currently validates predicate parameter and return types. Done means destructured parameters in type predicates are accepted and their narrowed types behave as shown without changing emitted JavaScript.