New Rule: discourage type assertions in favor of type guards

[adidahiya]

Name: no-type-assertion

• lines up with existing no-angle-bracket-type-assertion rule

Failing code:

function foo(myArg: any) {
  if (myArg.bar != null) {
    doTheThing(myArg as IBar);
  } else if (myArg.baz != null) {
    anotherThing(myArg as IBaz);
  } else {
    ...
  }
}

Passing code:

function foo(myArg: any) {
  if (isBar(myArg)) {
    doTheThing(myArg);
  } else if (isBaz(myArg)) {
    anotherThing(myArg);
  } else {
    ...
  }
}

function isBar(x: any): x is IBar {
  ...
}

function isBaz(x: any): x is IBaz {
  ...
}

[jskrzypek]

👍

[woutervh-]

👍

[JoshuaKGoldberg]

Relevant: discussion in #2040 on bringing in the equivalent rule from tslint-consistent-codestyle.

IMO it'd be better to name this type-assertion and allow configuring to:

• Ban type guardsassertions (default; enabled in tslint:all)
• Allow as Type (enabled in tslint:recommended, tslint:latest)
• Allow <Type>

Note that as ajafff mentioned, copied code without substantial changes needs to retain the original license.

[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! 👋