Tslint: New rule to warn against triple equals null

[adidahiya]

Almost always I want to compare against null and undefined, not just null. So === null is probably an error.

This rule should suggest to the user, Did you mean == null instead?

[myitcv]

:+1: for this

I'll add that if possible this rule should play nicely with the no-null-keyword rule. That is:

• if no-null-keyword is 'on'
  
  
  
  • === null should error because of the no-null-keyword rule
  
  
  • === undefined should error because of the rule proposed in this issue
  
  
• if no-null-keyword is 'off'
  
  
  
  • === null and === undefined should both error because of the rule proposed in this issue
  
  

Perhaps the title of the issue could be adjusted to reflect this?

[adidahiya]

@myitcv right now we don't have a way of communicating between rules to check if others are enabled. So I think the best path forward is to split your ideas up into two rules, no-triple-equals-null and no-triple-equals-undefined. See https://github.com/palantir/tslint/pull/1060/files#r58076945

[JamesDunne]

Since undefined is not a reserved keyword in JavaScript and could easily be redefined, please extend the a == undefined check to include the pattern a == void(0), a == void 0, a == void true or other void-expressions since void is actually a reserved keyword and will always return the undefined primitive value.

[richardm]

+1

[andy-hanson]

--strictNullChecks should catch the error. i.e.:

function len(x: string | null | undefined) {
    return x === null ? 0 : x.length; // Error: object is possibly 'undefined'.
}

[adidahiya]

I'd still like to have this rule for projects that haven't migrated to strictNullChecks yet

[JoshuaKGoldberg]

Note: per #4534, this issue will be closed in less than a month if no PR is sent to add the rule. If you _really_ need the rule, custom rules are always an option and can be maintained outside this repo!

[JoshuaKGoldberg]

💀 _It's time!_ 💀

TSLint is being deprecated and no longer accepting pull requests for new rules. See #4534. 😱

If you'd like to see this rule implemented, you have two choices:

• Recommended: Use typescript-eslint to lint your TypeScript code with ESLint ✅
• Legacy: Write it as a custom TSLint rule 🤷‍♂️

👋 It was a pleasure open sourcing with you!

_If you believe this message was posted here in error, please comment so we can re-open the issue!_

[JoshuaKGoldberg]

🤖 Beep boop! 👉 TSLint is deprecated 👈 _(#4534)_ and you should switch to typescript-eslint! 🤖

🔒 This issue is being locked to prevent further unnecessary discussions. Thank you! 👋