Lexical environment of functions in JavaScript

All external variables available to the function are called its Lexical Environment

In the following function example, two variables are available: num1 and num2, which are the lexical environment of our function:

let num1 = 1;
let num2 = 2;

function func() {
	// the function knows about the variables num1 and num2
}

The lexical environment itself is some kind of internal JavaScript object attached to our function. In this case, it can be represented as follows:

{num1: 1, num2: 2}

The value of any lexical environment variable is always equal to the current value of that variable:

let num1 = 1; // environment {num1: 1}
let num2 = 2; // environment {num1: 1, num2: 2}

// Let's change the variable num1:
num1 = 123; // environment {num1: 123, num2: 2}

function func() {
	
}

When we try to refer to some variable inside a function, this variable is first looked up among the local variables of the function and, if such a variable is not there, then it is looked up in the lexical environment of the function.