Functions

// Function Declaration
function myFunctionDeclaration() {
  console.log("Hello, world!");
}

// Function Expression
const myFunctionExpression = function() {
  console.log("Hello, world!");
}

// Arrow Function (syntactic sugar for Function Expression)
const myArrowFunction = () => {
  console.log("Hello, world!");
}

Differences

Function Declaration is hoisted, meaning it can be used before it is declared.

• Created as a property in the Variable Object (VO) of its context

Function Expression and Arrow Function are not hoisted, meaning they cannot be used before they are declared.

• Variable holds a reference to the function.

What is VO

VO stands for Variable Object.

A special object created for each execution context (like function or global scope).

It contains all the variables, functions, and arguments for that execution context.

Callback Functions

A callback function is a function that is passed as an argument to another function.

// Callback Function
function callbackFunction() {
  console.log("Hello, world!");
}

// Passing a Callback Function
someFunction(callbackFunction);

Methods like forEach, map, filter, reduce, etc. take a callback function as an argument.

array.map(callbackFunction);

In this case, the declaration is passed as an argument, not the function itself.

Passing Function as an Argument

const myFunctionDeclaration = function() {
  console.log("Hello, world!");
}

// Passing Function (callback)
someFunction(myFunctionDeclaration);

// Passing a Return Value of a Function
someFunction(myFunctionDeclaration());

// Passing an IIFE (passing a Function Return Value)
someFunction((function myFunctionDeclaration() {
  console.log("Hello, world!");
})());

// IIFE in this case has the same effect as passing a Function Return Value
someFunction(function() {
  console.log("Hello, world!");
})();

What is IIFE

IIFE stands for Immediately Invoked Function Expression.

It's a way to create a function and immediately execute it upon declaration.

(function() {
  console.log("Hello, world!");
})();

Difference between IIFE and Function Declaration

When passed to a function as an argument, IIFE is executed immediately while Function Declaration is not.

//Function Declaration
function myFunction() {
  console.log("Hello, world!");
}

// IIFE
(function() {
  console.log("Hello, world!");
})();

IIFEs cannot be used as a callback function because they are executed immediately.

• If you pass in a IIFE as an argument, it will pass the resulting value of the IIFE as an argument, not the function itself.