execution-context.md

chapter	27
pageNumber	260
description	Understanding Execution Context in JavaScript.

Execution Context in JavaScript

In JavaScript, an execution context is an environment where the code is evaluated and executed. It is a fundamental concept that helps manage the scope and behavior of variables and functions.

Types of Execution Context

There are three main types of execution contexts in JavaScript:

1. Global Execution Context: This is the default context where the code starts execution. It creates a global object (e.g., window in browsers) and sets up the global scope.
2. Function Execution Context: Created whenever a function is invoked. Each function has its own execution context.
3. Eval Execution Context: Created when code is executed inside the eval function.

Phases of Execution Context

Each execution context goes through two phases:

1. Creation Phase: In this phase, the JavaScript engine sets up the environment for the code to be executed. It involves:

   • Creating the this binding.
   • Setting up the scope chain.
   • Creating the variable object (variables, functions, and arguments).

2. Execution Phase: In this phase, the JavaScript engine executes the code line by line.

Example of Execution Context

Here's an example to illustrate how execution contexts work:

var globalVar = "I am a global variable";

function outerFunction() {
    var outerVar = "I am an outer variable";

    function innerFunction() {
        var innerVar = "I am an inner variable";
        console.log(globalVar); // Accesses global variable
        console.log(outerVar);  // Accesses outer variable
        console.log(innerVar);  // Accesses inner variable
    }

    innerFunction();
}

outerFunction();

Output:

I am a global variable
I am an outer variable
I am an inner variable

Explanation

1. Global Execution Context:

   • globalVar is created and assigned the value "I am a global variable".
   • outerFunction is created and stored in memory.

2. Function Execution Context (outerFunction):

   • outerVar is created and assigned the value "I am an outer variable".
   • innerFunction is created and stored in memory.

3. Function Execution Context (innerFunction):

   • innerVar is created and assigned the value "I am an inner variable".
   • The console.log statements access variables from their respective scopes.

Scope Chain and Lexical Environment

The scope chain is a list of all the variable objects that the currently executing code has access to. The lexical environment is the environment in which the code is written, and it determines the scope chain.

Example of Scope Chain

var a = 10;

function foo() {
    var b = 20;

    function bar() {
        var c = 30;
        console.log(a + b + c); // Accesses variables from all scopes
    }

    bar();
}

foo();

Output:

60

Explanation

1. Global Scope: Contains a.
2. Function Scope (foo): Contains b and has access to a.
3. Function Scope (bar): Contains c and has access to a and b.

Hoisting

Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their containing scope during the creation phase.

Example of Hoisting

console.log(hoistedVar); // Output: undefined
var hoistedVar = "I am hoisted";

hoistedFunction(); // Output: I am a hoisted function
function hoistedFunction() {
    console.log("I am a hoisted function");
}

Explanation

• hoistedVar is declared but not initialized during the creation phase, so it is undefined when accessed.
• hoistedFunction is fully hoisted and can be called before its declaration.