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Message
ReferenceError: "x" is not defined
Error type
What went wrong?
There is a non-existent variable referenced somewhere. This variable needs to be declared, or you need make sure it is available in your current script or scope.
Note: When loading a library (such as jQuery), make sure it is loaded before you access library variables, such as "$". Put the <script> tag that loads the library before your code that uses it.
Examples
Variable not declared
foo.substring(1); // ReferenceError: foo is not defined
The "foo" variable isn't defined anywhere. It needs to be some string, so that the String.prototype.substring() method will work.
var foo = 'bar'; foo.substring(1); // "ar"
Wrong scope
A variable needs to be available in the current context of execution. Variables defined inside a function cannot be accessed from anywhere outside the function, because the variable is defined only in the scope of the function
function numbers() {
var num1 = 2,
num2 = 3;
return num1 + num2;
}
console.log(num1); // ReferenceError num1 is not defined.
However, a function can access all variables and functions defined inside the scope in which it is defined. In other words, a function defined in the global scope can access all variables defined in the global scope.
var num1 = 2,
num2 = 3;
function numbers() {
return num1 + num2;
}
console.log(numbers()); // 5