Add strict-comparisons Rule

[AndreasGassmann]

PR checklist

• Addresses an existing issue: fixes Rule suggestion: Ban comparisons of non-primitives #3574
• New feature
  • Includes tests
• Documentation update

Overview of change:

This rule disallows the comparison of objects.

>, >=, <, <= are only allowed if the operands are of type number.
=== and !== are only allowed for operands of type number, string, boolean and enum or if either of the operands is null or undefined.

Is there anything you'd like reviewers to focus on?

This is my first tslint rule and in general the first time I took a closer look at the typescript internals.

I'm not sure if all the type checks are bullet proof. I'm pretty sure there's a better way to do those which I haven't figured out yet.

CHANGELOG.md entry:

[new-rule] strict-comparison

[AndreasGassmann]

Looks like there are some cases that I didn't think about. Some of the things I saw:

• Numeric enums should be allowed with >, <, <= and >=. (How can I find out if it's a numeric enum or not?)
• Some people do use >, <, <= and >= with strings. Should I add an option to allow that?

[JoshuaKGoldberg]

Posted a few comments on the rule name and that large conditional.

Love the test cases!

[AndreasGassmann]

I see a couple of things that are still open:

• Sometimes, enum comparisons seem incorrect (eg. looking at the failing circleci: lint step here). I was not able to reproduce this, so I'm not sure what's going on.

• < and > should be allowed between Numeric Enums, but not between String Enums (without the allow-string-comparison option). However, I'm not sure how to do that because it doesn't seem like those are different types in typescript.

• Rename allow-equal and allow-string-comparison because it's not clear what they do right now.
  allow-equal could be named allow-object-equal-comparison
allow-string-comparison could be named allow-string-order-comparison (I'm not sure how to call the "ordering" operators like >, <, >=, <=).

• Corner cases including any undefined and null have to be handled.

[JoshuaKGoldberg]

All right, this is in! Thanks so much @AndreasGassmann for getting started on the rule implementation and working earlier this year on the initial rounds of feedback. strict-comparisons will be available in the next TSLint version.

[rolftimmermans]

Feedback from a user who likes to use "extends": ["tslint:all"] to test new rules: I don't think this rule is correct in its current form.

Example code :

function foo(s: string | undefined) {
  if (s === undefined) return "none"
  return s
}

foo(undefined)

Lint error with this rule enabled:
foo.ts[2, 7]: Cannot use '===' comparator for type 'undefined'.

To me there seems absolutely nothing wrong with testing a variable of type string | undefined for equality with undefined.

[JoshuaKGoldberg]

Feedback from a user who likes to use "extends": ["tslint:all"]

Love it! 💖

Could you please file a new issue for feedback? Separate issues and PRs for different topics are easier to track and search for.

[rolftimmermans]

Could you please file a new issue for feedback? Separate issues and PRs for different topics are easier to track and search for.

Absolutely! Searching through the issues my experience seems similar or identical to #4774, so I'll leave it at that. Thanks!