A variable that is undefined means that it has been declared but has not been assigned a value. A variable that is null is explicitly assigned the value null, indicating that it has no value. An undeclared variable is a variable that has not been declared, and trying to access it will result in a ReferenceError.
- undefined: A variable that is declared but has not been assigned a value is undefined. For example, if you declare a variable with the "var" keyword but don't give it a value, it is undefined:
var x; console.log(x); // Output: undefined
- null: A variable that is explicitly assigned the value null has no value. This can be used to indicate that a variable that previously had a value, no longer has a value.
var y = null; console.log(y); // Output: null
- undeclared: A variable that has not been declared, and trying to access it will result in a ReferenceError.
console.log(z); // ReferenceError: z is not defined
An important thing to note is that the undefined and null are two different things and should not be used interchangeably. undefined generally means a variable has been declared but has no value, while null means a variable has been declared and has no value.
Also, Undeclared variables are those that do not use the var, let, const keywords while declaring them. They will be created in the global scope and can cause unexpected behavior.