Final answer:
In programming, a variable can be null, undefined, or undeclared. 'Null' is a variable that has been declared with no value, 'undefined' is a variable that hasn't been assigned a value, and 'undeclared' means the variable has not been created. Use equality checks and the typeof operator to determine the state.
Step-by-step explanation:
In programming, understanding the differences between null, undefined, and undeclared variables is crucial since they represent different states of variables.
Null is an assignment value that signifies that a variable does not point to any object or value. It is intentional to have a null value, implying that the variable has been declared but intentionally set to no value.
Undefined means that a variable has been declared but has not yet been assigned a value. It is the default value of variables that have just been declared or of function arguments that have not been provided.
Undeclared means that the variable has not been declared at all. Trying to access such a variable will result in a ReferenceError, as the variable is not known to the compiler or interpreter.
To check for these states:
- For null, you can directly check if the variable is equal to null.
- For undefined, you can use the typeof operator; if a variable is undefined, ‘typeof variable’ will return 'undefined'.
- For an undeclared variable, you can also use the typeof operator; despite not being declared, ‘typeof undeclaredVariable’ will still return 'undefined' instead of throwing an error.