1. Function Declaration

A function created with a function declaration is a Function object and has all the properties, methods and behavior of Function objects.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function#Description

function add(a, b) {
  return a + b;
}

2. Function Expression

Its like declaring a variable but using the function keyword.

const add = function(a, b) {
  return a + b;
}

3. Arrow Function Expression

An arrow function expression has a shorter syntax than a function expression and does not have its own this, arguments, super, or new.target. These function expressions are best suited for non-method functions, and they cannot be used as constructors.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

const add = (a, b) => {
  return a + b;
}

4. Concise Body Arrow Function Expression

An arrow function expression has a shorter syntax than a function expression and does not have its own this, arguments, super, or new.target. These function expressions are best suited for non-method functions, and they cannot be used as constructors.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

const add = (a, b) => a + b;

5. IIFE "iffy" or the Immediately-Invoked Function Expression

An IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined.https://developer.mozilla.org/en-US/docs/Glossary/IIFE

(function(a, b) {
  alert(a + b);
})(a, b);