JS-Lesson-1

• JS is a lightweight (doesn't use much memory from computer), cross platform, object oriented computer programming language.

• JS is one of 3 core technologies of web development

• JS can be used in different places:

  • Client-side: JS was traditionaly only used in the browser
  • Server-side: Thanks to node.js, we can use JS on the server as well.

What happens to our code?

• JS is always hosted in some environment, typically a browser. This is where JS runs. Such google chrome, Firefox etc...

• When we write our JS code and run it, behind the scenes:

  • Host where our JS code lives has a JS engine, that takes our code and executes it.

• 1st thing that happens:

  • Our code is parsed by a parser (reads our code line by line and checks if the syntax of the code is correct)
  • If we make mistakes, it throws an array and stops the execution
  • If everything is correct, then the parser produces a data stucture known as the "abstract syntax tree":

    • This is then translated into machine code (set of instructions that can be executed directly by computer processor)

    • Only when it is converted, it runs and does the work.

Execution Context:

• A container that stores variables, and in which a piece of our code is evaluated and executed

• The default context is the global context:

  • code that is not inside any function
  • associated with global object
  • in the browser, thats the window object

• So this means the global execution context is associated with the global object ( which is the window object ) in the browser.

• They get stacked as our code gets executed

How is the Execution Context created?

• When a function is called:
  • a new execution context is put on top of the execution stack.
  • this happens in 2 stages:

    • Creation phase:

      • Creation of the variable object
        • The argument object is created, containing all the arguments that were passed into the function.
        • The code is scanned for function declarations, for each function, a property is created in the variable object, pointing to the function.
        • Creation of the scope chain
        • Determine value of the 'this' variable

    • Execution phase:

      • The code of the function that generated the current execution context is ran line by line

Creation Phase:

• The argument object is created, which contains all the arguments that were passed into the function

• HOSTING:

  • Code is scanned for function declarations: for each function, a property is created in the variable object, pointing to the function.

  • The code is scanned for var declarations: for each var, a property is created in the variable object, and set to undefined