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javascriptvariablesundefined
Supreme Dolphin
Supreme Dolphin

Reputation: 2594

JavaScript variable === undefined not working

I have the function:

isset = function(obj){
  alert(obj !== undefined);
 }

Then when I do isset(defined_variable) where defined_variable has been declared, the alert box shows true but when I do isset(undefined_variable) where undefined_variable has not been declared, the alert box does not show at all, while I expected the alert box to show false. What am I doing wrong? I have tried using typeof but the results are the same.

Upvotes: 3

Views: 1239

Answers (3)

axelduch
axelduch

Reputation: 10849

When you dereference an undeclared variable (meaning you try to use a symbol that was never wrote before), you get a Reference error.

There are several ways to deal with this, but you can't determine what are local variables in javascript. Thus, your problem can be solved dynamically only for global variables, or a given scope object.

No functions could handle local scope dynamically.

You can prefix it with window if you are in a browser context (for more context see this SO answer about global object in javascript)

This would not modify your isset function code:

isset(window.undefined_variable)

There would be another way, which would need that isset function changes, but uses the same principle (still in browser context):

isset('undefined_variable_name_wrawppped_in_a_string')

function isset(name) {
    return name in window;
}

We can't really use typeof in isset, it's sad because it would be convenient since it doesn't throw a Reference Error when the variable was never declared. We could still use a form of eval to do it, but since I don't want us to go there, I won't implement it.

But now, what if you want to check several nested properties?

function isset (memoryPath, root) {
    var nodeNames = memoryPath.split('.');
    var expectedDepthReached = nodeNames.length;
    var depthReached = 0;
    var nodeName;
    var node = root;

    // we are gonna read it from left to right
    // using Array.prototype.pop()
    // reversing it will alow us to do so
    nodeNames.reverse();

    do {
        nodeName = nodeNames.pop();
        node = node[nodeName];

        if (node) {
            depthReached++;
        }
    } while (node);

    return depthReached === expectedDepthReached;
}

And an exemple:

window.myGlobals = {
    path: {
        to: {
            a: {
                variable: true
            }
        }
    }
};

isset('myGlobals.path.to.a.variable', window), // true
isset('myGlobals.path.foo', window) // false

Upvotes: 1

Madara's Ghost
Madara's Ghost

Reputation: 174977

That's because there's a difference between undefined and undeclared.

var foo; // foo is undefined.
// bar is undeclared.

console.log(foo === undefined); // true
try {
  console.log(bar === undefined);
} catch (e) {
  console.error('I had an error!'); // gets invoked.
}

foo = 'something';
console.log(foo === undefined); // false

Upvotes: 4

gurvinder372
gurvinder372

Reputation: 68393

but when I do isset(undefined_variable) where udefined_variable has not been declared, the alert box does not show at all, while I expected the alert box to show false

because it throws an error in your console (check your console) that this variable that you are comparing is not defined.

Uncaught ReferenceError: 'c' is not defined

i tried with isset( c ) and c was not declared first

Upvotes: 1

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