Use typeof in if statement expression to check variables 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 scopePreviously, 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 scopeSource: 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:
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