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.

Start with the motivating examples in the issue and compare the proposed !== assertion syntax with existing is and asserts return type syntax. The scope of supported operators and type expressions is intentionally unresolved, so done would require settling the syntax and semantics, implementing the type narrowing, and adding appropriate compiler coverage.