rule: no double-negation in boolean context

[michaelficarra]

Double negation (!!<expr>) is often used for casting to bool, but in a boolean context this is unnecessary. Boolean contexts are anywhere a value will be coerced to boolean.

• if(<bool>) ...
• <bool> ? ... : ...
• do ... while(<bool>)
• while(<bool>) ...
• for(...; <bool>; ...) ...
• Boolean(<bool>)
• new Boolean(<bool>)
• !<bool>

A similar proposal was recently submitted to jshint: jshint/jshint#1487.

[btmills]

Working on this.

The rule name no-double-negation doesn't seem right because double negation can be okay in some contexts. What about no-redundant-boolean-cast?

[michaelficarra]

That name sounds good to me.

[nzakas]

I'd go no-extra-Boolean-cast

We tend to use "extra" in a lot of rules

[btmills]

Couple of on-the-fly decisions I've made and hope you can confirm:

1. We'd also want to add <bool> && <bool> and <bool> || <bool> to this rule?
2. This would be considered an error and on by default?

[michaelficarra]

@btmills: First one: nope, those values are both observable. Remember, && and || do not coerce the result to bool. Second: yes, I'd say so.

[btmills]

@michaelficarra Thanks, I forgot to consider the result of the expression.

Should have a PR here in a few minutes.