declaring a variable within conditional expressions (ternary operator)

Question

Is it possible to declare the variable within a conditional expression?

for example: The code below return a syntax error (because I've declared the variable x within the conditional expression?).

var a = document.getElementById("userData");
var d = a.value;
function() {
(d.length>15)?(
 alert("your input was too long")):(
 var x = parseInt(d).toString(2), 
 a.value=x 
 );
 }

obviously this can be fixed by simply adding var x; outside the statement, but is it possible for variables to be declared here?

No. Then would something like var a = (var b!=undefined) ? (var c=1) : (var d=2); be legal — davidkonrad
– davidkonrad, Commented Sep 28, 2013 at 21:27
@delnan honestly, just curiosity. i don't think it has any benefits other than shortening my code by a character or two. — Liam B
– Liam B, Commented Sep 28, 2013 at 21:31
@LiamB You should aim for readable, not short. Needlessly short code is often too clever to be easily understood by the next person who has to maintain your code. (Or even by yourself when you come back to the code a month later.) — cdhowie
– cdhowie, Commented Sep 28, 2013 at 21:31

T.J. Crowder · Accepted Answer · 2013-09-28 21:27:03Z

9

Is it possible to declare the variable within a conditional expression?

No. var is a statement, and the operands to a conditional expression are expressions. The language grammar doesn't allow it. Thankfully.

answered Sep 28, 2013 at 21:27

T.J. Crowder

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Comments

cdhowie · Accepted Answer · 2013-09-28 21:25:44Z

1

You can do this with an immediately-invoked function:

(d.length>15)?(
    alert("your input was too long")):
    (function(){
        var x = parseInt(d).toString(2);
        a.value=x;
    }())
);

But note that the x variable will not exist outside of the inner function. (I can't tell whether you want it to exist after the expression is evaluated or not.)

answered Sep 28, 2013 at 21:25

cdhowie

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3 Comments

T.J. Crowder Over a year ago

+1 Yeah. You can (provided you don't need the variable outside the expression), but you ought not. :-)

cdhowie Over a year ago

@T.J.Crowder A self-invoking function expression is not "nuts," it is the fundamental way to create a new scope in JavaScript, since the language does not have block scope. In this case it serves that purpose. (What "ought not" be done in this case is using the ternary operator instead of a proper conditional.)

T.J. Crowder Over a year ago

@ cdhowie: It isn't nuts per se, and anyone who does significant JavaScript work does it regularly. This use of it would be nuts. :-) And of course, the variable wouldn't be available outside of it anyway, as you've quite correctly pointed out.

Manos Nikolaidis · Accepted Answer · 2016-02-10 15:49:44Z

0

No. But you can initialize it with undefined and set it with condition.

function Test()
{
    d = 25.6654;
    var x = (d.toString().length > 15) ? parseInt(d).toString() : undefined;

    alert(typeof x === "undefined");
}

Then you can work with if(typeof x == "undefined") //do something

edited Feb 10, 2016 at 15:49

Manos Nikolaidis

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answered Sep 28, 2013 at 21:25

Dvir

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