JavaScript check if undefined | Example code

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Use the in operator for a more robust check of variables if undefined or not in JavaScript. Or you can use typeof like this:

if (typeof something != "undefined") {
    // ...
}

JavaScript check if undefined

Simple example code to finding out whether a variable has been declared regardless of its value, using the in operator is the safest way to go.

// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"

If you are interested in knowing whether the variable hasn’t been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string:

<!DOCTYPE html>
<html>
<body>

  <script>
   if (typeof myVar == 'undefined')
   {
    alert("Hello variable")
   }

</script>

</body>
</html> 

Output:

JavaScript check if undefined

Source: stackoverflow.com

Do comment if you have any doubts or suggestions on this JS if the topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version

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