当我尝试将 return 与三元运算符一起使用但不与 if 语句一起使用时，出现 UnexpectedToken 错误？

Question

so I noticed something very strange. When I try to exit a function with return keyword in the if statement it works well but when I try to do this with the ternary operator it throws UnexpectedToken error (expected expression, got keyword 'return'). Why does this happen?

My code:

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

monthName(mnr);

But with if statement:

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

monthName(mnr);

It works fine. So why can't I use return keyword with ternary operator but with if statement it works?

Answer 1

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).

---

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

Answer 2

You can only include expressions in ternaries. If you look at the MDN docs , you'll notice that there are only string, bools, etc after the condition. The closest solution only returns or only alerts:

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

or

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

Answer 3

In a ternary the return cannot be an operand. Therefore when the interpreter reads the : operator in the ternary expression it expects a valid value to follow but instead sees the reserved keywork return causing the program to throw and error.

Answer 4

first of all, this is not like in math, you can not do

0 < nr < 13

because this will try to do the first comparison 0 < nr and then the result of this, it will be compared to 13

so basically if nr is positive, it will always be true

0 < nr // true

therefore

true < 13 // true

you can check this in the following code:

 console.log(0 < 5 < 13, " -> should be true. good") console,log(0 < 5000 < 13, " -> should be false! but it is true")

so in order to do the things correct, you need to concatenate the expressions using AND

(0 < nr) && (nr < 13)

now, to the things you asked, basically as the other responses say, you are trying to use an statement where you can only use expressions , if you read the documentation , it says you need to provide an expression to execute .

so, to fix your issue, you can return the whole ternary expression.

 function monthName(nr) { return (0 < nr) && (nr < 13)? alert("Month: " + nr): false; } monthName(5);

or if you want a shorthand:

const monthName = nr => (0 < nr) && (nr < 13) ? alert("Month: " + nr) : false;

monthName(5);