Usefulness of the comma operator in comparisons

Question

I saw a comparison like the following in a question on SO:

(pNum != ('2','3','4','5','6','7','8','9'))

The OP has been trying to check if a number falls in a certain range but this code is inaccurate as it will always compare with right most value inside the brace(i.e. 9) This means when pNum = 2 the comparison will return true and not false as was expected by OP who was expecting it to work like inArray or in.

My question is whether this sort of comparison is going to be useful in any real case in any scenario?

`var arr = ['2','3','4','5','6','7','8','9']; return arr.includes(pNum);` — gurvinder372, Feb 07 '18 at 09:01
@Jamiec this question has nothing in common with the duplicate — pwolaq, Feb 07 '18 at 09:03
@pwolaq you sure? ref: "My question is whether this sort of comparison is going to be useful " — Jamiec, Feb 07 '18 at 09:03
The expression does not make any sense. Please point to the question / answer where you have seen this — smnbbrv, Feb 07 '18 at 09:11
@smnbbrv its at the bottom of the code here https://stackoverflow.com/questions/48656050/javascript-unexpected-outcome-of-character-removal-code — techie_28, Feb 07 '18 at 09:13
@techie_28 It is literally the same question. With the same answer. — Jamiec, Feb 07 '18 at 10:37
well okay,Please tell me how to accept that "it is the duplicate of that question as an answer"? — techie_28, Feb 09 '18 at 05:05
this is a closer possible duplicate in the context of the question https://stackoverflow.com/questions/3561043/what-does-a-comma-do-in-javascript-expressions — techie_28, Feb 09 '18 at 05:36

score 0 · Answer 1 · answered Feb 07 '18 at 10:17

My question is whether this sort of comparison is going to be useful in any real case in any scenario?

No. As you observe, the comparison only compares the last item inside the bracket. So all it can accomplish is to confuse the reader.

If you intend to compare a variable with a set of values, you could use array#includes or array#indexOf >= 0. Something like:

console.log(['2','3','4','5','6','7','8','9'].includes('2'));
console.log(['2','3','4','5','6','7','8','9'].includes('6'));
console.log(['2','3','4','5','6','7','8','9'].includes('9'));

// IE 
console.log(['2','3','4','5','6','7','8','9'].indexOf('2') >= 0);
console.log(['2','3','4','5','6','7','8','9'].indexOf('6') >= 0);
console.log(['2','3','4','5','6','7','8','9'].indexOf('9') >= 0);

Usefulness of the comma operator in comparisons

1 Answers1