how to check efficiently if variable has declared

Question

I have many js methods that uses global variables like:

function DoSomething() {
    var val=$('#'+clientId).val();
    .
    .
    .
}

DoSomething uses clientId variable. The problem is that clientId is not always declared and therefore I get an exception of undeclared variable.
I have many places like this and add if (typeof clientId == "undefined") check everywhere is a lot of work and code duplication (duplication of the check). Is there I can workaround it? for example use a method that returns null if the variable is not declared? (In such way I can control on the return value in case of an undefined for example)

You can create such a method, implementing if (typeof variable === 'undefined'). It's a great solution in my opinion. — iMoses
– iMoses, Commented Mar 28, 2012 at 14:56

jfriend00 · Accepted Answer · 2012-03-28 14:54:05Z

1

It seems to me that your code is just busted if you have code paths that attempt to use a global variable before it is defined. Either you need to give it a default value, you need to reorder the loading or execution of your code or you need to prevent code from running when your environment hasn't been initialized.

It is possible to initialize a variable if and only if it hasn't already been defined like this:

var clientId = clientId || "foo";

You could put this at the top of any module that uses it. This would initialize it with whatever value you want if and only if it wasn't already initialized.

You could also build yourself an accessor function that you would use instead of directly accessing the variable like this:

// put this somewhere early in the JS loading process
function getClientId() {
    if (typeof window.clientId !== "undefined") {
        return(window.clientId);
    } else {
        return("foo");   // whatever default value you want to return here
    }
}

// use it like this:
function DoSomething() {
    var val=$('#'+getClientId()).val();
    .
    .
    .
}

Or, you can make a global function that will test it:

function isClientId() {
    return(typeof window.clientId !== "undefined");
}


// use it like this:
function DoSomething() {
    if (isClientId()) {
        var val=$('#'+clientId).val();
        .
        .
        .
    }
}

answered Mar 28, 2012 at 14:54

jfriend00

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4 Comments

Naor Over a year ago

The problem in var clientId = clientId || "foo"; is that when clientId is defined then it became undefined.

jfriend00 Over a year ago

@Naor - that is not correct. var clientId = clientId || "foo"; cannot make clientId be undefined. It assigns "foo" to clientId only if clientId is a falsey value like undefined, null, 0, false, etc... It does not and cannot make it become undefined. If you are seeing that, then there is something else going on in your code that you would have to show us.

Randall Flagg Over a year ago

@Naor Your example is not 100% correct. You are running your code var bbb='defined'; after you execute the other code. Put it in the function or remove the var keyword and it will work. I can't explain why because this is a crazy language but it works.

jfriend00 Over a year ago

@Naor - your second variable declaration inn your jsFiddle is a local variable declaration and thus a separate from your global variable because it's inside a function. var statements inside a function create a local variable, not a global variable. If you want to do it that way, then you have to explicitly refer to the global variable like this: jsfiddle.net/jfriend00/fwTk5.

KooiInc · Accepted Answer · 2012-03-28 16:59:30Z

0

Just an idea: all variables are properties of a namespace, up to the global namespace (in a browser, that's window). A property that's not assigned evaluates to undefined. So if you want to know if a variable is assigned, you can use if (property in namespace). If it concerns a global variable in a browser: if (clientId in window). Now you can make a function out of that:

function exists(label, namespace){
  namespace = namespace || window;
  return namespace[label] || false;
}
// possible usage
function DoSomething() {
 var val=$('#'+( exists('clientId')||0) ).val();
 // ...
}

edited Mar 28, 2012 at 16:59

answered Mar 28, 2012 at 15:08

KooiInc

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Comments

Roy Dictus · Accepted Answer · 2012-03-28 14:52:38Z

0

Global variables are evil and are to be avoided if at all possible.

In the example you give, it would be much better to declare client ID as a parameter, like so:

function DoSomething(clientId) {
    var val=$('#' + clientId).val();
    .
    .
    .
}

This way, you are not affected by issues with global variables and there are no side effects (value changes caused by other methods) to worry about.

Good luck!

answered Mar 28, 2012 at 14:52

Roy Dictus

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1 Comment

Roy Dictus Over a year ago

I love it when people who are just reaching out to help others then get slammed for it. You could have just added your link without the nasty comment, that would have been so much nicer. I am not a parrot, by the way, I have 29 years of programming experience and know the ropes, thank you. Have a nice day.

Royi Namir · Accepted Answer · 2012-03-28 14:54:27Z

0

try :

var isExists =clientId || false;

but be careful

if ClientId==0 it will show you false.

answered Mar 28, 2012 at 14:54

Royi Namir

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Comments

Redtopia · Accepted Answer · 2012-03-28 14:56:00Z

0

I use:

if (typeof (foo) != 'undefined')
   alert ('foo is defined');

answered Mar 28, 2012 at 14:56

Redtopia

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1 Comment

Redtopia Over a year ago

I think this is the most foolproof way to do it... but I agree with beardtwizzle. I often use the "same type" equivalency "===" like: if (foo === true) or if (foo === 'text')

kitti · Accepted Answer · 2012-03-28 14:57:33Z

0

If the variable itself is declared (as opposed to just having an undefined value), then you can simply use ( clientId || 'default value' ). This requires that you have var clientId; or something similar in the global scope (which is bad practice, BTW).

If you do not have a guarantee that clientId has been declared somewhere, then using the above construct will still throw an undefined variable error. In that case, you can use this: ( typeof clientId == 'undefined' ? 'default value' : clientId ).

answered Mar 28, 2012 at 14:57

kitti

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Comments

Eric · Accepted Answer · 2012-03-28 15:14:06Z

0

While i like the answers, I go the simple way... Just declare var clientId; on top of your script. If you don't pass it any values then :

var clientId;
if(!clientId){
     //will reach here cause declared but with no value
}

but

var clientId;
clientId = 132;
if(!clientId){

}else{
      //will reach here cause it has a value
}

so

function DoSomething() {
      if(clientId){

Would work

answered Mar 28, 2012 at 15:14

Eric

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