check used state when using ternary assignment

[MohammedEssehemy]

fixes ternary assignment with destructuring

render() {
    const {foo} = true ? this.state : {};
    return <SomeComponent bar={foo} />;
}

would result in unused state field foo, although the field is used.

[ljharb]

I don't think this should be considered used in the general case - with true, obviously, the entire ternary reduces to const { foo } = this.state; - but if the condition isn't "always true", then the state is unused in some cases, so it's appropriate for the rule to warn about it.

You should be unconditionally destructuring out of state.

[MohammedEssehemy]

If I retype the same condition as
if (condition) { const { foo } = this.state; }
the linter would accept this;
but if I type:
const {foo} = condition? this.state : {};
I will get unused field state, although the two are the same and the state field is unused if the condition is falsy.

[ljharb]

I would expect both to be considered unused. Let’s fix that bug instead.

[MohammedEssehemy]

Actually, I don't agree with that, I don't consider this as bug, there are punch of cases where you need to conditionally get field from state.
IMO the linting rule applies on static analysis of code, so is a field will be sometimes used based on dynamic conditions, it should be considered as used.

[ljharb]

I can't think of any - you should always be unconditionally extracting from state, and then conditionally using those values.

[MohammedEssehemy]

compare this

you should always be unconditionally extracting from state, and then conditionally using those values.

const { foo } = this.state;
if(condition) {
 // use foo
}

with

if(condition) {
const { foo } = this.state;
 // use foo
}

you first comment still applies for both cases.

but if the condition isn't "always true", then the state is unused in some cases, so it's appropriate for the rule to warn about it.

but, in the first form when condition is false, I created a variable (foo) although I might not use it at all, I think the second approach would be better for resources.

[ljharb]

Performance is the least important concern; creating variables is free; and the cost of a single property get is effectively free as well.