undefined value in JS

In Javascript, undefined does not mean some empty value or undeclared variable.

If a variable has a value undefined, it means it has been declared in the code, it has been hoisted in the memory space/execution context but has not been assigned any value. Thus undefined is a special value in javascript, all variables declared in the code are assigned this undefined value in the 1st Hoisting phase of code execution cycle. ( Javascript code is executed in two phases - see Javascript Code Execution topic).

If your code uses some undeclared value, the javascript execution will throw an error 'Uncaught Reference Error: <<var>> is not declared'

Since undefined is a special variable/value in javascript, we can write boolean expressions comparing a variable to undefined.

console.log(a)

/// ------- output
/// above code would trigger an error when the code runs
/// Uncaught Reference Error: a is not declared

var a;
console.log('>>>>', a)

if (a === undefined) {
 console.log('>>>> a has a value of undefined');
} else {
 console.log('>>>> a has some defined value');
}

/// ------- output
/// above code would output below
/// >>>> undefined
/// >>>> a has a value of undefined

When an undeclared variable is assigned a value, the JS engine (in non-strict mode) would automatically add the variable to the global object in the 1st Hoisting phase of code execution.

a1 = 3;
console.log('a1=', a1)

/// ------- output
/// for the above code  (assuming 'use strict' is not enabled),
/// the JS engine (in non-strict mode), would add 'a' to the global object
/// a1=3


/// in strict mode, for the above the JS engine will throw 
/// Uncaught Reference Error: a1 is not declared