use-isnan fails when use NaN in || operator

[timocov]

Bug Report

• TSLint version: 4.3.0
• TypeScript version: 2.2.1
• Running TSLint via: CLI

TypeScript code being linted

declare var a: number | undefined;
console.log(a || NaN);

with tslint.json configuration:

{
  "rules": {
    "use-isnan": true
  }
}

Actual behavior

Found an invalid comparison for NaN: a || NaN

Expected behavior

No errors

[ajafff]

But that's not a comparison.
Your code does basically the following: If a is truthy (a !== 0 && a !== undefined && !isNan(a)) log a, else log NaN.
There's nothing wrong in this piece of code and nothing the use-isnan-Rule could detect.

[sbj42]

I believe @timocov would agree with you, that use-isnan should not complain about that code, but it does. I was able to reproduce it by adding the following lines to rules/use-isnan/test.ts.lint:

// no violation for logical operators
x = 0 || NaN;
x = 0 && NaN;

Then ran the tests and got:

// no violation for logical operators
x = 0 || NaN;
    ~~~~~~~~  [Found an invalid comparison for NaN: 0 || NaN]
x = 0 && NaN;
    ~~~~~~~~  [Found an invalid comparison for NaN: 0 && NaN]

The implementation of use-isnan has an exception only for the simple assignment operator, but TypeScript has many other operators which are not "comparison" operators. Many of them would be very silly to use with NaN, but nevertheless the rule does not correctly limit its scope.

[ajafff]

Ah, sorry. I misread the opening post.

@sbj42 you already analyzed the problem. Fixing this should not be that difficult. Would you like to fix it or should I go for it?

[sbj42]

I'd like to fix it. I'll give it a try later today.

[sbj42]

I believe this is now resolved.