Enhancement: [@typescript-eslint/no-unnecessary-condition] Detect unnecessary comparison to falsy values

[Samuel-Therrien-Beslogic]

Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the rule's documentation

https://typescript-eslint.io/rules/no-unnecessary-condition/

Description

I would like unnecessary falsy values checks when the variable is already checked for truthiness in the same condition chain to be raised. I often see newer devs not realize that their condition is redundant.

Fail

const myVar = '' as string | number;

if (myVar && myVar !== '') {
  // do something
}

if (myVar && myVar !== 0) {
  // do something
}

if (myVar && !!myVar ) {
  // do something
}

Pass

const myVar = '' as string | number;

if (myVar) {
  // do something
}

Additional Info

No response

[bradzacher]

Unfortunately this is not really achievable because the types do not convey this information.
You cannot remove specific cases from the primitive types:

declare const x: string;
// typeof x === string

if (!x) { return };
// typeof x === string

if (x !== '') { return }
// typeof x === string

So in order for this to be a lint rule, we would need to invent some method for tracking this by tracking a type as it progresses through a function's control-flow. This is obviously not really possible for us to build.

Something like negated types in TS (microsoft/TypeScript#29317) would help solve this (because TS could refine the type to like string & not '', but without it - we're SOL.

[Samuel-Therrien-Beslogic]

Thank you for the explanation and reference. I went and upvoted it.