Execution Context

Execution Context is the environment in which JavaScript code is executed. It determines which variables, functions, and objects are accessible at a specific moment of execution.

Types of Execution Context

1. Global Execution Context (GEC)
   • Created once when the script starts.
   • Defines global objects (window, globalThis) and the global scope.
   • Any code outside of functions is executed here.
2. Function Execution Context (FEC)
   • Created every time a function is called.
   • For each function, its own Lexical Environment and this value are formed.
   • After the function completes, the context is destroyed (unless closed in a closure).
3. Block Execution Context (BEC)
   • Appeared with ES6 for let, const, class constructs.
   • Created for blocks { ... }, loops, if, try/catch.
   • Provides block scope.

Execution Phases

1. Creation Phase
   • A variable and function object (Variable Environment) is created.
   • Hoisting is performed: function and variable declarations are registered in memory.
   • The value of this is determined.
2. Execution Phase
   • Code is executed line by line.
   • Values are assigned to variables, function calls and expressions are executed.

Call Stack

• Each new context is placed in the Call Stack.
• When a function completes, its context is removed from the stack.
• The top element of the stack always corresponds to the current execution context.

function a() { console.log('A'); }
function b() { a(); console.log('B'); }
b();
// Stack trace: [GEC] → [b()] → [a()]

Key Ideas

• Each function call creates a new Execution Context.
• The context defines scope, this, hoisting, and links to the outer environment.
• Call Stack manages the order of execution and return of functions.
• After a function completes, the context is removed, but can be preserved through a closure.