Disallow case NaN, switch(NaN), indexOf(NaN), and lastIndexOf(NaN) (use-isnan)

[feross]

https://eslint.org/docs/rules/use-isnan

This is a proposal to modify the use-isnan rule.

Current rule setting:

use-isnan: "error"

Proposed rule setting:

["error", {"enforceForSwitchCase": true}]

The switch statement internally uses the === comparison to match the expression's value to a case clause.
Therefore, it can never match case NaN. Also, switch(NaN) can never match a case clause.

Set "enforceForSwitchCase" to true if you want this rule to report case NaN and switch(NaN) in switch statements.

Examples of incorrect code for this rule with "enforceForSwitchCase" option set to true:

/*eslint use-isnan: ["error", {"enforceForSwitchCase": true}]*/

switch (foo) {
    case NaN:
        bar();
        break;
    case 1:
        baz();
        break;
    // ...
}

switch (NaN) {
    case a:
        bar();
        break;
    case b:
        baz();
        break;
    // ...
}

Examples of correct code for this rule with "enforceForSwitchCase" option set to true:

/*eslint use-isnan: ["error", {"enforceForSwitchCase": true}]*/

if (Number.isNaN(foo)) {
    bar();
} else {
    switch (foo) {
        case 1:
            baz();
            break;
        // ...
    }
}

if (Number.isNaN(a)) {
    bar();
} else if (Number.isNaN(b)) {
    baz();
} // ...

[mightyiam]

Makes sense to me. If anyone is breaking by this that might be a bug rather than good code.

[feross]

No ecosystem impact. This is shipping in standard 16!

[feross]

There's a new option: "enforceForIndexOf": true which disallows the use of indexOf and lastIndexOf methods with NaN. Let's enable it as well.