Uncaught ReferenceError (not defined) when checking === undefined

javascript comparison undefined comparison-operators

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That error is saying that there is no such variable (it was never declared), not that its value is undefined.

To check whether a variable exists, you can write typeof someGlobal

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Author by

WBT

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Updated on June 04, 2022

Comments

WBT almost 2 years

I have the following code:

simpleExample.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Simple example</title>
</head>
<body>
    Open the Console.
    <script src="js/simpleExampleJS.js"></script>
</body>
</html>

js/simpleExampleJS.js:

MyObject = {
    COMPUTER_GREETING: "Hello World!",
    hello: function() {
        console.log(MyObject.COMPUTER_GREETING);
    }
};

checkSomeGlobal = function() {
    if(someGlobal === undefined) {
        console.log("someGlobal is undefined & handled without an error.");
    } else {
        console.log("someGlobal is defined.");
    }
};

MyObject.hello();
checkSomeGlobal();

When I run this, I get:

Hello World!
Uncaught ReferenceError: someGlobal is not defined
at checkSomeGlobal (simpleExampleJS.js:9)
at simpleExampleJS.js:17

(The first line of output generally indicates that the code is loading and running).

MDN indicates that a potentially undefined variable can be used as the left-hand-size of a strict equal/non-equal comparison. Yet when checking if(someGlobal === undefined) that line of code produces an error because the variable is undefined, instead of making the comparison evaluate to true. How can I check for and handle this undefined variable case without an error?

WBT about 6 years

To summarize the solution in this answer, it's to change the problematic comparison on line 9 to read if(typeof someGlobal === "undefined"). Thank you for your answer!