Suggestion: stricter operators

Currently operators like “+” are defined such that they match their semantics in JS. The below are all allowed by the compiler and produce the shown values, even with --strictNullChecks on.

• 2 + 'a' => "2a"
• null + 'a' => "nulla" (!)
• 2 - null => 2

I propose letting users opt in (maybe via some --strictOperators) to strict operator behavior. Concretely I think this means:

• restrict +, and += to just number and string, e.g. for the former only declare

  function +(a: number, b: number): number;
  function +(a: string, b: string): string;
  

• restrict - and -= to just number

(any should continue to work as normal, of course.)

Relevant spec section: https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#419-binary-operators

See also “Expression operators” in the strictNullTypes change: https://github.com/Microsoft/TypeScript/pull/7140 and in particular this rationale: https://github.com/Microsoft/TypeScript/pull/7140#issuecomment-186432250

This would fall under of “stricter” TypeScript, https://github.com/Microsoft/TypeScript/issues/274 .

[basarat]

number + string is actually quite common in JavaScript land and will probably not happen as it moves the convenience - type safety slider too much towards safety.

null + string should be disabled as a part of strictNullChecks.

Note:

A number of wat things are disabled in TypeScript e.g. [] + [] (valid JavaScript, produces “”) is an error in TypeScript. Similarly "hello" + 1 is allowed (like I mentioned) but "hello" - 1 is an error 🌹

[myitcv]

@basarat I really struggle when the argument “because that’s how Javascript does it” is applied. Particularly in cases like this where I think the cognitive load on the developer is increased by decisions to stick to the Javascript way. Not because it’s unclear how + behaves, rather that is creates cognitive dissonance with the rest of the type system:

let s: string;
let n: number;

s = n;            // ERROR: number is not assignable to string
n = s;            // ERROR: string is not assignable to number

let res = s + n;  // OK: really?

I understand that + is defined by the spec to work on combinations of number and string and so has well-defined behaviour, but given the example above I think it’s more confusing than useful for it to have been defined in this way (the Javascript way). Particularly when explicit conversion is so simple and more readable.