Uncaught ReferenceError: controller is not defined

Uncaught ReferenceError: the controller is not defined is a common error message that can occur in JavaScript. It usually occurs when you try to access a variable or function that has not been defined or is out of scope.

An example where this error occurs: For example, the following code will result in an Uncaught ReferenceError: the controller is not defined error because the variable “controller” has not been declared.

Javascript

<script>
    function myFunction() {
        console.log(controller);
    }

    myFunction();
</script>

Output:

Uncaught ReferenceError: controller is not defined

Approach: The solution to this error is to make sure that all variables are declared before they are used. In the example above, the code can be modified to declare the “controller” variable before using it.

Example 1: Below is the code illustration of the above approach:

Javascript

<script>
    let controller = "Hello, world!";

    function myFunction() {
        console.log(controller);
    }

    myFunction();
</script>

Output:

Hello, world!

Example 2: Use the let keyword to declare the variable within a block of code

Javascript

<script>
      function myFunction() {
          let controller = "Hello, world!";
          console.log(controller);
      }

      myFunction();
</script>

Output:

Hello, world!