Negated types

Sometimes it is desired to forbid certain types from being considered.

Example: JSON.stringify(function() {}); doesn’t make sense and the chance is high it’s written like this by mistake. With negated types we could eliminate a chance of it.

type Serializable = any ~ Function; // or simply ~ Function
declare interface JSON {
   stringify(value: Serializable): string;
}

Another example

export NonIdentifierExpression = ts.Expression ~ ts.Identifier

[kelsny]

I want this feature in some way or another, maybe even as a marker type like ThisType.

I have a “language” for matching tree-like structures:

{
    type: "NOT_NUMBER";
    value: not number;
}

In this language of mine, I support not, which matches all types but its operand. So not string and not number would disallow strings and numbers.

I’ve got most of this excellently (over engineered to hell) typed:

fantasyLibrary.match(compiledExpression, myTree, (match) => {
    // match is of type { type: "NOT_NUMBER"; value: unknown; }
});

The match parameter is as close as possible to what the original source expression describes. However, because TypeScript doesn’t have any way of representing the negation of types, I am forced to type anything that uses not as unknown, which isn’t as great as I would’ve liked it to be.

You can see the actual file here, on line 3, where I have no better type to give other than unknown 😦

If this was a thing in TypeScript, I would be able to provide even more accurate types in the callback of my library function.

[dead-claudia]

Could this be re-opened in light of #29317?