What is an undeclared variable?

• An undeclared variable is created when you assign a value to an identifier which has not been previously created using var, let or const.
• Are defined globally outside of the current scope.
• Can only occur outside of strict mode. If enabled, strict mode will throw a ReferenceError instead.

// undeclared variable
// assigned global scope
x = 1;

console.log(x) // 1

'use strict'

// Throws ReferenceError: x is not defined
x = 1;

What is an undefined variable?

• An undefined variable in Javascript is a variable which has been declared but has no value assigned to it.
• It has the type undefined.

var x;

console.log(x) // undefined

What is the difference between null and undefined?

• undefined is the value of a variable which has not been assigned a value.
• null has to be assigned to a variable.
• null has the type Object

var x; // x is undefined
var y = null; // y is null

console.log(x == y); // true
console.log(x === y); // false

• Using the abstract equality operator ==, null and undefined will be equal.
• Using the strict equality operator, ===, null and undefined are not equal as x is of type undefined and y is of type Object.