JavaScript ReferenceError - variable is not defined

This JavaScript exception variable is not defined and occurs if there is a non-existent variable that is referenced somewhere.

Message:

ReferenceError: "x" is not defined

Error Type:

ReferenceError

Cause of Error:

There is a non-existent variable that is referenced somewhere in the script. That variable has to be declared or make sure the variable is available in the current script or scope.

Example 1: In this example, the variable(val1) is being accessed from the outside of the function, So the error has not occurred.

function sum() {
    let val1 = 2;
    let val2 = 3;
    return val1 + val2;
}
console.log(val1);

Output:

ReferenceError: 'val1' is not defined

Example 2: In this example, the variable(GFG) is not defined, So the error has occurred.

GFG.substring(2);

Output:

ReferenceError: 'GFG' is not defined