Can For-In loop result an undefined or null?

Question

This is my code:

const a = function(obj) {
  for (let key in obj) {
    if (!obj.hasOwnProperty(key)) {
      continue;
    }
    console.info(key.split('_'));
  }
};
a({a_b: 123});

I thought there is no problem at all but SonarQube gives me a critical error:

TypeError can be thrown as "key" might be null or undefined here.

The word key in key.split('_') is highlighted. Indicating variable key can be undefined/null here.

I tried to pass in something like {[undefined]: 123}, and the variable key becomes a string "undefined" instead of real undefined.

Hence. I am wondering will the key be undefined/null in any possible situation? Or is it just a False Positive?

Here is a screenshot:

@Aᴍɪʀ That was a typo, the console.info is inside the for loop and the error still exists. — SCLeo, Nov 03 '16 at 02:04
`key` is defined at updated Question. Not certain what you are trying to determine? — guest271314, Nov 03 '16 at 02:05
Hmmm. Maybe there's a bug in SonarQube. I've never worked with it. Keys in objects are always strings, so there's no way `key` become `undefined`. — Aᴍɪʀ, Nov 03 '16 at 02:08
@Aᴍɪʀ `{}` would have an `undefined` key. Or, at least log `undefined` at `console` if passed to `for..in`. — guest271314, Nov 03 '16 at 02:08
@guest271314 no, that's just the output of the command as it gets evaluated by console. — Aᴍɪʀ, Nov 03 '16 at 02:09
@Aᴍɪʀ What is occurring at `for (let j in {}) console.log(j)` ? — guest271314, Nov 03 '16 at 02:10
@guest271314 try `for (let j in {}) console.log('x:'+j)`, see what you got. — Takahiro, Nov 03 '16 at 02:12
@Aᴍɪʀ He already deleted them... Anyway, thanks for indicating the fact that "Keys in objects are always strings". I definitely read that sentence somewhere else before and now I just recognized it. It should be a bug of SonarQube then. — SCLeo, Nov 03 '16 at 02:18
The actual bug in your code is the [`obj.hasOwnProperty` call](http://stackoverflow.com/a/13296897/1048572). — Bergi, Nov 03 '16 at 09:12
@SCLeo Because it doesn't work if your object has an `hasOwnProperty` property (which is usually more probably than `obj` inheriting some enumerable properties). It needs to be `Object.prototype.hasOwnProperty.call(obj)`, or you should just omit it completely. — Bergi, Nov 04 '16 at 16:16
@Bergi Please, please don't give SonarQube more ideas about things to complain about. — , Nov 04 '16 at 16:26
@Bergi Oh yea, that is true... But I don't think I will ever have that problem. The reason I add hasOwnProperty here is to prevent prototype pollution... — SCLeo, Nov 05 '16 at 03:22
@torazaburo Actually, that would be one good thing more linters should complain about :-) — Bergi, Nov 05 '16 at 18:29

score 8 · Accepted Answer · answered Nov 03 '16 at 07:40

8

That's a known bug in the SonarQube JavaScript analyser which was fixed a few months ago. You should upgrade to the latest version of the JavaScript plugin.

answered Nov 03 '16 at 07:40

Pierre-Yves

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Can For-In loop result an undefined or null?

1 Answers1