When should I use try/catch instead of functions to declare non global variables in JavaScript?

Question

Consider following codes:

try{
     throw undefined;
}
catch(someVariable){
    someVariable = 10;
    // do whatever you want with someVariable
    // someVariable will not be a global object at all. (without help of any function scope)
}
// someVariable is no longer valid here

Why some people use this syntax instead of functions when they don't want to declare global variables?

Additional notes:

I've seen this syntax a lot, but the most important one is Google traceur

try {
    throw undefined;
  } catch (a) {
    a = 10;
  }

that it is generated because of the following ecma script 6 syntax:

{
    let a = 10;
}

Google traceur is a ECMA Script 6 parser on older browsers that currently have no support for new JavaScript features.

I can't say that I've seen that much, but I thought try/catch blocks was something one should generally avoid, so seems like a bad idea to me, but I could be wrong ? — adeneo, Mar 10 '14 at 13:25
Maybe I haven't had enough coffee yet, but I don't understand what this is trying to accomplish. — Evan Davis, Mar 10 '14 at 13:35
@Mathletics It seems a cumbersome way to create a different context — Pablo Lozano, Mar 10 '14 at 13:36
"*Some people use this syntax*" - please link a few examples. We've never seen this. — Bergi, Mar 10 '14 at 14:26

score 2 · Accepted Answer · edited May 23 '17 at 11:57

Well, there is a difference: Why do catch clauses have their own lexical environment?

try {
     throw undefined;
} catch(someVariable) {
    someVariable = 10; // someVariable will not be global
    var someOtherVariable = 5; // but any declared 'var' will be
}
// someVariable is no longer valid here
console.log(someOtherVariable); // still 5

As you can see, this is just the same behaviour as you would have with

{
    let someVariable = 10;
    var someOtherVariable = 5;
}
console.log(someVariable, someOtherVariable);

It's basically a hack by the Traceur transpiler to create lexical environments for single variables in EcmaScript 5. Probably Google also optimized their browser to recognize this pattern, which explains why it's quite fast on Chrome.

In manually written code, this definitely is to be a bad practise - and without comments, no one knows what this does. Use an IEFE for readability.

Pablo Lozano · Answer 2 · 2014-03-10T14:28:06.233

~~It seems a really bad practice. Exceptions usually slow the execution, and making the system to raise an error an then re-use the error variable is overkill~~. UPDATE: For me this is a bit shocking: it seems the code is faster when using a try-catch in Chrome, but slower in Firefox and faaaar slower in IE 10: Here is a test I've just created.

Anyway, I think the proper way is using IIEFs like this:

(function () {
    var someVariable=...
    ...
})();

which it's more elegant. Another difference is only the variable with the error is local, any other variable created on the catch block will be global.

score -1 · Answer 3 · edited Jun 27 '14 at 10:26

-1

You should use try/catch when it is possible that the operations will cause some error (IOException,...). If there is an error in try it will do what it is in catch. If there is no possibility of error use function.

edited Jun 27 '14 at 10:26

BenMorel

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answered Mar 10 '14 at 13:31

gon

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The OP is not asking why you use try catch – epascarello Mar 10 '14 at 14:07

When should I use try/catch instead of functions to declare non global variables in JavaScript?

3 Answers3