Request: positive and negative numbers as a subtype of number #41666

Search Terms

number, numbers, positive numbers, negative numbers, unsigned

Suggestion

Unlike the proposals requesting explicitly sized values, such as i32, I am requesting a type that differentiates between negative numbers and positive numbers.

Use Cases

This has immediate use practically everywhere.
There are a whole lot of times where numbers are intended to be positive numbers.

For example, indexing arrays should always be positive:

type Array<T> = {
    [key: positive_number]: T
};

Why the hell would you want a negative index!?

Furthermore, plenty of built-in functions just throw errors for using negatives.

Some examples:

42.0.toString(-2);

BigInt.asIntN(-2, -1n);

new Array(-37);

If anything, they can prevent using negative constants in places where there should be positives.

Granted, there are plenty of places where generic number stills fits just fine. Also, I have no clue what we do about NaN.

And lastly, it is unlikely that negative literals are ever used erroneously, but negative runtime values can make it into places where... you wouldn't want them to be.

Examples

Introduce a way to denote that a number or bigint should be positive or negative.

let x: positiveNumber = 3; // fine, constant 3 is positive
...
x = -5; // error, constant "-5" is not a subtype of positiveNumber

x = 24654.36546; // not negative, perfectly fine

x = -7 as unknown as positiveNumber; // hands off, that's on the developer

x = Infinity; // questionable, but still positive

x = -0; // ???

x = NaN; // ???

Checklist

My suggestion meets these guidelines:

• [ x ] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [ x ] This wouldn't change the runtime behavior of existing JavaScript code
• [ x ] This could be implemented without emitting different JS based on the types of the expressions
• [ x ] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [ x ] This feature would agree with the rest of TypeScript's Design Goals.