Ternary Expression with "Defined" Returns "Expression" Instead of Value

Ternary expression with defined? returns expression instead of value?

defined? is binding to amount ? amount : r(1,4) so it is equivalent to:

defined?(amount ? amount : r(1,4))

You probably want:

defined?(amount) ? amount : r(1,4)

Actually, odds are that amount || r(1,4), or amount.nil? ? r(1,4) : amount would better match what you want, since I think you don't want this:

1.9.3p194 :001 > defined?(amount)
 => nil 
1.9.3p194 :002 > amount = nil
 => nil 
1.9.3p194 :003 > defined?(amount)
 => "local-variable" 

...in which case @c would be nil - the value of the defined variable.

Why can't we have return in the ternary operator?

The ternary operator evaluates to an expression and expressions can't contain a return statement (how would that behave if you were to assign the expression to a variable?). However, you could very well return the result of a ternary operator, i.e. return condition? returnValue1 : returnValue2;

On your specific point, I don't see why you would like to return. It looks like you're trying to do something only if a condition is fulfilled. A simple if statement would probably be more adequate there.

UnexpectedToken error when I try to use return with ternary operator but not with if statement?

Why does this happen?

Because return is a statement, and you can't put statements where only expressions are allowed.¹ The conditional operator's operands must be expressions, not statements. (You'd have the same problem with throw, just FWIW, though there's talk of adding a throw operator... Or with while or for or any other statement.)

The if is probably your best bet.

If you were really, really intent on just using a single expression, you could put the return at the start so the single expression is what follows it:

function monthName(nr) {
    return 0 < nr < 13 ? alert("Month: " + months[nr]) : false;
}

Note that that relies on the fact that alert returns undefined in order to maintain the two return values you were providing in your if example (undefined if the alert was shown, false if not). Interestingly, the spec doesn't explicitly say that alert returns undefined but I think we can safely accept that it's implicitly specified (not least because the implicit return of any non-arrow JavaScript function is undefined unless return <value> is used).

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¹ (You can do the opposite; JavaScript has the concept of the "expression statement" which means any expression is also a valid statement.)

Alternative for ternary operator in the case a ? a : b without repetition (JavaScript)

You could take a default approach with a logical OR ||. This returns a, if this values is truthy, otherwise the default value of b.

a || b

Why do ternary operator and if statement return different results?

In the ternary version, it is interpreting your default as default(DateTimeOffset), the other output expression type in the conditional. Then it is interpreting the ternary as a whole as a nullable, which will never be null.

In the second case, your return is using default(DateTimeOffset?), from the declared return type.

In this case, you may want to use an explicit type in the default expression, or: just use the second form and add a comment (and ideally a unit test), so nobody "fixes" it in the future.

Ternary operators and Return in C

return is a statement. Statements cannot be used inside expressions in that manner.

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