Rule Proposal: prefer-optional-chaining

Please describe what the rule should do:

The new rule warns obj && obj.prop-like notations to suggest obj?.prop.

What new ECMAScript feature does this rule relate to?

Optional chaining

What category of rule is this? (place an “X” next to just one item)

[X] Suggests an alternate way of doing something (suggestion)

Provide 2-3 code examples that this rule will warn about:

//✖ BAD
obj && obj.prop
obj != null ? obj.prop : undefined
obj && obj[key]
obj != null ? obj[key] : undefined
func && func()
func != null ? func() : undefined
if (func) func()
if (func) { func() }

//✔ GOOD
obj?.prop
obj?.[key]
func?.()

Why should this rule be included in ESLint (instead of a plugin)?

obj && obj.prop-like notation is a popular practice. This rule will help people to learn the new syntax Optional Chaining. This is about a language feature rather than third-party platforms or libraries.

Are you willing to submit a pull request to implement this rule?

Yes.

[bradzacher]

I’d agree that the code definitely not good and that nobody should ever write it.

But people do, and people rely on this sort of behaviour.

https://github.com/typescript-eslint/typescript-eslint/issues/2963 https://github.com/typescript-eslint/typescript-eslint/issues/1893

And there are other problems as well, like

declare const foo: null | {value: string};

const value = foo && foo.value;
// typeof value === null | string

if (value === null) {
  // code here to handle null
}

// after rule fix

const value = foo?.value;
// typeof value === undefined | string

if (value === null) {
  // this code is now unreachable
} 

https://github.com/typescript-eslint/typescript-eslint/issues/1820

This code is much more reasonable, relying on === null is a pretty common thing (though I always recommend == null as it handles both null and undefined, and IMO the two should always be handled the same). Fixing this case will break code, and reporting here can be considered a false positive.

What I’m saying is that without type information and complete, omnipresent knowledge of the usage of the value - this rule will always have false positives, and a fixer is not always safe.

Suggestion fixers are an alternative, but from my experience users will just apply them without thinking, and will break their code.

I just wanted to drop my learnings to save you guys from making the same mistakes.

[aladdin-add]

friendly ping @mysticatea : are you still working on this? it seems very useful! 👍