What is the difference between undeclared and undefined variables?

If you’ve ever struggled to understand the difference between undeclared and undefined variables, fear no more – we’ve got you covered.

Undeclared variables, in layman’s terms, are variables that have not been properly introduced to the program. It’s like showing up to a party without an invitation – you’re not on the guest list, so you’re not allowed in! In code, this means the variable hasn’t been declared in the current scope. But like a party crasher, an undeclared variable can still cause a commotion by being assigned a value. And if that happens, the variable becomes global and suddenly everyone knows its name!

On the other hand, undefined variables are like a blank canvas – they exist, but they haven’t been painted yet. They’ve been declared in the program, but they haven’t been given a value. It’s like having a present under the Christmas tree without knowing what’s inside – it’s there, but you don’t know what it is until you unwrap it. And just like a wrapped present, undefined variables have a default value – in most programming languages, that default value is “undefined.”

// Example of an undeclared variable
function foo() {
  x = 10; // This variable 'x' is not declared in this scope
  console.log(x); // Output: 10
}

foo();
console.log(x); // Output: 10 (x is now declared globally)

// Example of an undefined variable
var y;
console.log(y); // Output: undefined

var z;
console.log(z); // Output: undefined

In the first example, x is not declared within the scope of the foo function, but it is assigned a value of 10. Since it is not declared, it will become a global variable after it is assigned a value. Accessing x outside of the function will output 10. In the second example, y and z are declared but not assigned a value, so their default value is undefined. Accessing them will output undefined.

So there you have it, folks! Undeclared variables are the party crashers of the programming world, while undefined variables are the wrapped presents waiting to be opened. Now that you know the difference, you’ll be the life of the programming party – and you won’t even need an invitation, Happy Coding.