Javascript：“抛出undefined”是否安全？

Question

In the function below, when the condition fails I want the situation to be handled as a simple error (no details needed). Just out of curiosity, is it OK and safe to write throw undefined ?

function splitYearMonth (YM) { // Returns ["yyyy-mm", yyyy, mm]
  try {
    var o = YM.match(/^(\d{4})\-(0[1-9]|1[012])$/);
    if (o != null) {
      return  [o[0], parseInt(o[1], 10), parseInt(o[2], 10)];
    } else {
      throw undefined;
    }
  } catch (e) {
    return [undefined, undefined, undefined];
  }
}

Answer 1

Yes, it is safe to do so.

The ECMAScript 5.1 specification says :

The production ThrowStatement : throw [no LineTerminator here] Expression ; is evaluated as follows:

1. Let exprRef be the result of evaluating Expression .
2. Return (throw, GetValue( exprRef ), empty).

ECMAScript 6 uses the same terms .

undefined is definitely an expression, so it can be thrown. You can see an example in this fiddle .

That said, throwing undefined may not be a good idea from a maintainability standpoint, because doing so leaves you with no information at all about the cause of the exception. Throwing a string would arguably be a better solution:

var o = YM.match(/^(\d{4})\-(0[1-9]|1[012])$/);
if (o != null) {
    return [o[0], parseInt(o[1], 10), parseInt(o[2], 10)];
} else {
    throw "unrecognized date format";
}

---

Update: On second thought, unless the no details needed clause in your question means you're not telling us the whole story, you only need control flow, not exception handling. You only have to write:

function splitYearMonth(YM) {  // Returns ["yyyy-mm", yyyy, mm]
    var o = YM.match(/^(\d{4})\-(0[1-9]|1[012])$/);
    if (o != null) {
        return [o[0], parseInt(o[1], 10), parseInt(o[2], 10)];
    } else {
        return [undefined, undefined, undefined];
    }
}

Exception handling can be expensive, and it is usually not recommended to use this facility for control flow (since we're discussing Javascript and not, say, Python).

Answer 2

The syntax of throw is:

throw expression;

As undefined is a valid expression, it is safe to do this, although it is generally good practice to return a sensible error message, for example:

throw "Failed to split year and month for the given input"