Skip to main content

Execution context

Execution context is the environment in which the JavaScript code is executed. It defines the state of the JavaScript engine, including variables, objects, and functions, at a particular point in time.

Types of Execution Context​

There are three main types of execution contexts in JavaScript

Global Execution Context​

  • Created when the JavaScript code is initially run.

  • It is the default or base execution context.

  • In the browser, it is associated with the global object (i.e., window).

Function Execution Context​

  • Created whenever a function is called.

  • Each function call gets its own execution context.

  • Contains arguments, local variables, this value, and the function’s specific scope chain.

Eval Execution Context​

Created when the eval() function is used to execute code. Code executed inside eval() runs in its own execution context.

Phases​

Each execution context goes through two main phases.

Creation Phase​

The creation of the variable object (VO), the scope chain, and setting the value of this. Hoisting occurs: variables and function declarations are moved to the top of their scope.

Execution Phase​

The code is interpreted and executed (assignments happen and functions are invoked).

Components of Execution Context​

Variable Object (VO)​

Contains function arguments, inner variable and function declarations.

In the global context, it’s the global object. In a function, it’s the activation object (AO).

Scope Chain​

Consists of the current execution context’s VO and its parent execution contexts’ VOs. Determines the visibility of variables and functions.

this Keyword​

Special value that depends on how the function was called (e.g., as a method, constructor, or globally).