https://github.com/alok722/namaste-javascript-notes/tree/master/notes

Execution Context

When the JavaScript engine scans a script file, it makes an environment called the Execution Context that handles the entire transformation and execution of the code.

Everything in JS happens inside the execution context. Imagine a sealed-off container inside which JS runs. It is an abstract concept that hold info about the env. within the current code is being executed.

• In the container the first component is memory component and the 2nd one is code component
• Memory component has all the variables and functions in key value pairs. It is also called Variable environment.
• Code component is the place where code is executed one line at a time. It is also called the Thread of Execution.
• JS is a synchronous, single-threaded language
  • Synchronous:- In a specific synchronous order.
  • Single-threaded:- One command at a time.
• When a JS program run, a global execution context is created.
• Javascript manages code execution context creation and deletion with the the help of Call Stack.
• Call Stack is a mechanism to keep track of its place in script that calls multiple function.
• Call Stack maintains the order of execution of execution contexts. It is also known as Program Stack, Control Stack, Runtime stack, Machine Stack, Execution context stack.
• in memory creation phase it assigns undefined and puts the content of function to function's memory

Hoisting is a concept which enables us to extract values of variables and functions even before initialising/assigning value without getting error and this is happening due to the 1st phase (memory creation phase) of the Execution Context.

Shortest JS Program, this keyword and windows

• The shortest JS program is empty file. Because even then, JS engine does a lot of things. As always, even in this case, it creates the GEC which has memory space and the execution context.
• JS engine creates something known as 'window'. It is an object, which is created in the global space. It contains lots of functions and variables. These functions and variables can be accessed from anywhere in the program. JS engine also creates a this keyword, which points to the window object at the global level.
• In different engines, the name of global object changes. Window in browsers, but in nodeJS it is called something else. At global level, this === window
• If we create any variable in the global scope, then the variables get attached to the global object.