Support `!==` operator in `is` and `asserts` return type syntax

🔍 Search Terms

unknown, assert, narrow, undefined, "is not"

✅ Viability Checklist

• 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 our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

This looks like something that would have been asked before, but it's hard to search for.

Right now asserts is a pretty magical syntax that lets you do:

function assert(proposition: unknown): asserts proposition {
    if (!proposition) {
        throw new Error()
    }
}

function foo(value: unknown): {} | null {
    assert(value !== undefined);
    return value;
}

However, trying to express a more specialized function doesn't work

function assertNotUndefined<T>(value: T): asserts value is Exclude<T, undefined> {
    if (value === undefined) {
        throw new Error();
    }
}

function foo(value: unknown): {} | null {
    assertNotUndefined(value);
    return value;  // Doesn't work
}

This is because Exclude<unknown, undefined> is still just unknown, since unknown is not just an alias for {} | null | undefined. Assuming that won't change, how else can one express assertNotUndefined? Well, if we take the original assert function and try to retroactively rationalize its syntax as this

function assert(proposition: unknown): asserts proposition;

when called as

assert(proposition !== undefined)

(hand-waving) just specializes to this

function assert(proposition: unknown): asserts proposition !== undefined;

I don't care to bikeshed over whether the syntax should be asserts proposition is not undefined, whether the LHS or RHS operands can be flipped, whether != or other operators should exist, or if any type expressions other than primitive literals like undefined should be supported, so I'll leave that to the comments.

📃 Motivating Example

function assertNotUndefined(proposition: unknown): asserts proposition !== undefined;

function foo(value: unknown): {} | null {
    assertNotUndefined(value);
    return value;  // Works!
}

💻 Use Cases

1. What do you want to use this for?
   A function that can exclude undefined can be composed with other functions, e.g. array.filter(isNotUndefined)
2. What shortcomings exist with current approaches?
   There's no clean way to define such a function without ugly conditionals to handle unknown.
3. What workarounds are you using in the meantime?
   Rewrite code in more verbose style, e.g. manually construct a new array using a for loop instead of array.filter.

Beginne mit den motivierenden Beispielen in der Issue und vergleiche die vorgeschlagene !==-Assertion-Syntax mit der vorhandenen Syntax für Rückgabetypen mit is und asserts. Der Umfang der unterstützten Operatoren und Typausdrücke ist absichtlich noch ungeklärt; die Umsetzung würde daher voraussetzen, die Syntax und Semantik festzulegen, die Typverengung zu implementieren und eine angemessene Compiler-Abdeckung hinzuzufügen.