Understanding 'typeof' in JavaScript

In JavaScript, the typeof operator is used to return a string indicating the type of the unevaluated operand. For instance, if you apply typeof to an undefined variable, the result will be 'undefined'. This fact is indicated in the quiz question, where typeof undefined would indeed return 'undefined'.

Let's illustrate this with an example:

let variable;
console.log(typeof variable)

This script will output 'undefined' as the 'variable' has been declared but not defined yet.

Alternatively, you could directly use the typeof operator with 'undefined':

console.log(typeof undefined)

Again, the output will be 'undefined' as the type of the operand, which is 'undefined', is simply returned by the typeof operator.

So, why does JavaScript have 'undefined' as a type? Well, in JavaScript, if a variable has been declared but no value has been assigned to it, then the value of that variable is 'undefined'. It's a way for JavaScript to acknowledge that the variable exists, but it doesn't have a value yet.

It is important to understand how JavaScript handles types as it helps to anticipate the behavior of your program and reduce errors. For instance, knowing the type of a variable can be useful when debugging, as it might reveal why certain operations are not working as anticipated.

Moreover, understanding how 'undefined' works in JavaScript paves the way for best practices. Developers should always strive to ensure variables are properly initialized before using them. It's poor practice to leave variables uninitialized as it can lead to tricky-to-diagnose bugs in your code.

JavaScript is a dynamically-typed language which provides a level of flexibility, but also places responsibility on developers to manage types wisely. So always remember to declare and define your variables properly to prevent unexpected surprises from the 'undefined' type.