JavaScript if not undefined | Example code

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Use typeof in if statement expression to check variable not undefined in JavaScript. In JavaScript, null is an object. There’s another value for things that don’t exist, undefined.

Where typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.

But, to check if a variable is declared and is not undefined:

if (yourvar !== undefined) // Any scope

Previously, it was necessary to use the typeof operator to check for undefined safely, because it was possible to reassign undefined just like a variable. The old way looked like this:

if (typeof yourvar !== 'undefined') // Any scope

Source: stackoverflow.com

JavaScript if not undefined

Simple example code check if a JavaScript variable is NOT undefined. Remember, undefined is an object in JavaScript.

<!DOCTYPE html>
<html>
<body>
  <script>
    var msg = "Hi";

   if(typeof msg !== "undefined")
   {
    alert("Hi. Variable is defined.");
  } 

</script>
</body>
</html> 

Output:

JavaScript if not undefined

Do comment if you have any doubts or suggestions on this JS if statement 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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