Hoisting in JavaScript

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of thier scope before code execution. However, hoisting does not mean that the declarations are physically moved to the top. Instead, JavaScript's execution context first scans the entire code for declarations and register them in the memory before executing the code.

This means we can reference a variable or function before it is declared in the code.

How Hoisting works in Variables

• Varibale hoisting with var

  var are hoisted but are initialized as undefined

  Example 1:

  console.log(a); // undefined
  var a = 1;
  console.log(a); // 1
  

  Example 2:

  var a;
  console.log(a); // undefined
  
  a = 1;
  console.log(a); // 1
  

• Varibale hoisting with let and const

  Varibale declared with let and const are hoisted but are not initialized. They remain in a temporal dead zone (TDZ) from the start of the block until thier declaration is encountered. Accessing them before declaration results in a ReferenceError.

  Example:

  console.log(x); // ReferenceError: Cannot access 'x' before initialization
  let x = 10;
  
  console.log(y); // ReferenceError: Cannot access 'y' before initialization
  const y = 12;
  

How Hoisting works in functions

• Function Declarations

  JavaScript hoists the entire function declaration to the top, making it callable anywhere within its scope.

  Example:

      hello(); // Hello, World!
  
      function hello() {
      console.log("Hello, World!");
  }
  

• Function Expressions

  Function expressions, including arrow functions, behave differently

  Example:

      hello(); // TypeError: hello is not a function
  
      var hello = function() {
      console.log("Hello, World!");
  }
  

  In this example hello is declared using var, which is hoisted and initialized with undefined. So, if we try to invoke hello gives "TypeError".

Hoisting in Different Scopes

Scope	var Hoisting	let and const Hoisting
Global	Hoisted, initialized as undefined	Hoisted, but in TDZ (ReferenceError)
Function	Hoisted, initialized as undefined	Hoisted, but in TDZ (ReferenceError)
Block	Hoisted, but ignores block scope	Hoisted, but in TDZ (ReferenceError)

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