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JavaScript- Scopes and Closures

Scope

  • The scope is the set of variables, objects, and functions you have access to.

Create Scope 
  • Use functions
Declaring Variables
  •  If you do write operation without declaring variable -> work fine
    • foo = 10; console.log(foo);  -> 10
  • If you do read operation without declaring variable -> doesn't work fine
    • console.log(foo) -> syntax error
Window Object
  • Inbuilt object provided by JavaScript itself
  • Every global variable and functions are part of window object
JavaScript is both compilations and interpreted language
  • Compilation: register variables, method names, arguments in global scope
  • Interpretation: assign values to global scope and execute the code
  • global scope problem: undeclared function inside the variable it goes to global scope
Hoisting
  • It doesn't matter where you declare your variable (In the compilation step it include all variables automatically before the interpretation)
Strict Mode
  • To get strict mode: "use strict"
  • It doesn't allow use variables without declaring it before. 
Closure
  • Function remembers it's declaration scope at every stage when it executes
Callback
  • setTimeout(function,time)
  • The function you want to execute after waiting defined time(milliseconds)
Important Facts
  • JavaScript is not block scoping language: {} doesn't create blocks in JavaScript
  • IIFE: Immediately Invoke Function Expressions
    • Execute anonymous functions directly
  • When executing functions each time new copy of local variables are produce


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