New rule: no mixed logical operators

I’m proposing a rule that warns when logical operators like && and || are mixed. The purpose is to avoid errors like the one fixed here:

if (a && a.b === 1 || a.b === 2) {
}

In this example, we are mixing && and ||. We are probably expecting not to enter inside the if block if

• a is falsy
• a.b is neither 1 nor 2

but what the actual operation will be is (a && a.b === 1) || a.b === 2. Meaning, if a is undefined, then we will be evaluating a.b === 2, which will create an error.

If we had wrapped our different conditions in parentheses a && (a.b === 1 || a.b === 2), then this would have resulted in the wanted behavior.

Fail

if (a && a.b === 1 || a.b === 2) {
}

a && b || c
a || b && c

Pass

a && b && c
a || b || c
a && (b || c)
a || (b && c)

I think the rule is pretty generic and can help avoid “easy” mistakes, and therefore has it’s place in ESLint. I don’t mind adding it to a custom plugin otherwise. I’d be happy to write the rule 😃

[mysticatea]

• As it caused a bug of ESLint before, a mix of logical expressions can confuse people. So I have ever seen “enclose mixed logical expressions” in some code style guides in Japan. I think this rule is pretty important.
• Indeed, guessing developer’s intention is very difficult, but detecting a mix of logical expressions is easy. I made a prototype: http://astexplorer.net/#/7os6gj7FJO

[michaelficarra]

@nzakas Yes, my concern was that this rule only applied to logical operators. Now that it has been re-envisioned to lint all binary operators, it’s fine.