JavaScript variable === undefined not working

Question

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.

Answer 1

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

Answer 2

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

Answer 3

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