Operator '<' cannot be applied to types 'Number' and 'number'

[DarrenDanielDay]

Bug Report

🔎 Search Terms

Number object
number primitive
binary operator <
ts2365
TypeScript 5.0-dev

🕗 Version & Regression Information

• This changed between versions 4.9.5 and 5.0.0-dev.20230210

⏯ Playground Link

Playground link with relevant code

💻 Code

const a = new Number(1);
const b = 2;
console.log(a < b)

🙁 Actual behavior

In TypeScript 5.0.0-dev.20230210, it reports error ts2365, but in TypeScript 4.9.5, it doesn't.

🙂 Expected behavior

I'm not sure whether this is a bug of 5.0-dev or a breaking change that which will be introduced in 5.0. If it's a breaking change by design, maybe it should be included in #51362? If not, it should not report any error since it's valid JavaScript code.

Actually, I run into this error when I try to extend Number.prototype for utility:

interface Number {
    [Symbol.iterator](): Generator<number>;
}

Number.prototype[Symbol.iterator] = function*() {
    for (let i = 0; i < this; i++) yield i;
    //              ^^^^^^^^ ts2365
}

for (const i of 10) console.log(i);

[MartinJohns]

This is mentioned in the release notes:
Breaking Changes: Forbidden Implicit Coercions in Relational Operators

You can fix this by adding an explicit coercion: Number(this)

[Andarist]

Actually, I run into this error when I try to extend Number.prototype for utility:

This code is cursed 🤣

This is mentioned in the release notes:
https://devblogs.microsoft.com/typescript/announcing-typescript-5-0-beta/#forbidden-implicit-coercions-in-relational-operators

Yeah, it is - but this isn't about comparing a number to a non-number or about comparing a number to a union containing a number and other stuff. It is actually comparing a number to an instance of a Number class. This probably should be supported 🤔

[MartinJohns]

This code is cursed 🤣

Absolutely agreed. I'd definitely smack whoever submitted code like this in a code review.

It is actually comparing a number to an instance of a Number class. This probably should be supported 🤔

I guess. We'll see what the team has to say. I'd argue it's a rare corner case and technically it is coercing to number.

[DarrenDanielDay]

You can fix this by adding an explicit coercion: Number(this)

Yes, it works well, but the implicit conversion seems to be harmless, since it's just a comparison between Number and number. I think TypeScript should just report error about the dangerous implicit conversions like string to number, rather than safe conversions.

[Andarist]

The Number case was even caught in my PR (see a comment here) but Ryan Cavanaugh (@RyanCavanaugh) didn't respond with a yes/no answer if that should be corrected back then. And then the PR just landed :D

[DarrenDanielDay]

This code is cursed 🤣

Absolutely agreed. I'd definitely smack whoever submitted code like this in a code review.

Absolutely I will also not submit such code in teamwork🤣. It's just utility code for leaning purpose, to show how to use prototype, generator function and iterator protocol in JavaScript.

On the other hand, is the type Number/ Number object impractical in teamwork? If so, it's reasonable to ignore the corner case in that PR.
Here's a relevant issue I've found, and it was opened years ago: #2361. I think it's time to reach a clear answer for it in 5.0. I'd see how the team would say.

[RyanCavanaugh]

#2361 is in a tough spot because we'd really like it to happen, but Date just completely breaks any attempt at unifying the codepaths due to its carve-out for hint behavior

[Josh-Cena]

DarrenDanielDay You should definitely coerce this to a number before iterating. Consider this:

Number.prototype[Symbol.iterator] = function* () {
  for (let i = 0; i < this; i++) yield i;
}

const v = Number.prototype.valueOf;
Number.prototype.valueOf = function () {
  console.log("coerced");
  return v.apply(this, arguments);
}

for (const i of 10) console.log(i);
// Logs "coerced" 10 times!

This is consistent with how built-in methods normalize input—always coerce at the boundary to minimize number of calls to user code.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.