Detect if Javascript variable is undefined or null

When working with JavaScript, it’s important to be able to check if a variable is either undefined or null. This is important for preventing errors in your code and ensuring that your program behaves as expected.

Here’s a simple example of how to detect if a JavaScript variable is undefined or null:

“`html

var x;

if (typeof x === ‘undefined’ || x === null) {
document.getElementById(“demo”).innerHTML = “Variable x is either undefined or null”;
} else {
document.getElementById(“demo”).innerHTML = “Variable x has a value”;
}

“`

In this example, we declare a variable “x” but do not assign it a value. We then use an if statement to check if the variable is either undefined (using the typeof operator) or null. If it is, we update the content of the paragraph element with the id “demo” to indicate that the variable is either undefined or null.

It’s important to note that using the typeof operator to check if a variable is undefined is more reliable than simply using a comparison with “undefined”, as “undefined” is not a reserved word in JavaScript and can be redefined. Therefore, using typeof ensures that we are accurately checking for the undefined value.

By using this simple check, you can ensure that your JavaScript code handles undefined and null variables gracefully, preventing unexpected errors and ensuring the robustness of your program.