Allow composite values to be used in type predicates

Allow composite values to be used in type predicates

🔍 Search Terms

Is, composite type, complex type, composite value, complex value, type predicate, type guard

✅ 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

Type predicates allow you to define functions that can tell the TypeScript compiler if a parameter is of a certain type.
I suggest that this be extended to allow type predicates to narrow the type of composite objects involving parameters.
That is, the left-hand-side of is types should be allowed to include object literals.

📃 Example

Say you have a type that involves a subset of another type:

type MyType = {property1: "static text", property2: "Hello" | "World"};

A type predicate for this would always require passing in the property1 property, even though it’s unnecessary.
Now, you can create a type predicate that only takes the value of property2, but still gurantees the result type:

function foo(text: string): {property1: "static text", property2: text} is MyType {
    return ["Hello", "World"].includes(text);
};

I’ve run into the problem of being unable to do this while developing my website.
I had a tagged union:

type A = {t: "A", value: "X" | "Y"};
type B = {t: "B", value: string};
type U = A | B;

And I wanted to get a value of type A from user input, but default back to a B type if the value wasn’t
To do this, I hoped to be able to do this:

function zit(x: string): {t: "A", value: x} is A {
    return ["X", "Y"].includes(x);
};
function gob(x: string): U {
    return zit(x) ? {t: "A", value: x} : {t: "B", value: x};
};

No files, tests, or entry points are named. Start by reviewing the issue’s type-predicate examples and the compiler’s existing type-guard behavior; the change would be complete when composite object types are accepted in predicates and the shown narrowing scenarios work without changing emitted JavaScript.