checking a variable value using an OR operator

Question

So, a junior programmer on my team today wrote the following piece of code:

if(status === (\"incomplete\" || \"unknown\"))

Which is obvious

Answer 1

In JavaScript, the || operator returns its first operand if it evaluates to true (i.e. it is not false, null, undefined, "", or 0), and its second operand otherwise.

In the first case, ("incomplete" || "unknown") always evaluates to "incomplete", since it evaluates to true.

The entire condition then becomes:

if (status === "incomplete")

Which explains the behaviour you are observing.

Answer 2

("incomplete" || "unknown") will return "incomplete" which is than compared to status.

Answer 3

But what I can't explain is why exactly status === ("incomplete" || "unknown") wouldn't work

That's because the expression in the parenthesis is evaluated first. The non-empty string incomplete is truthy, so the OR-expression ("incomplete" || "unknown") yields "incomplete" and only that is then compared with your status variable.

To shorten the condition, there are many ways including arrays of values, regular expression test etc.