Functions are nothing but a block of reusable code. used to simplify the whole codebase. let us discuss with an example and realize the syntax.

//write a function to geet someone: 
const greet = function(name) {
    console.log("Hello, " + name + "!");
};
greet("javascript"); 
// Output: Hello, javascript!

here in the above block of code "name " is the parameter and "javascript" is known as arguments. Both can be one or more than one.

In ES6 a new form was introduced known as arrow function:

//write a function to add tow number in js
const add = ((a,b)=>{
     return a+b;
})
console.log(add(2,3));
//output 5

Arrow functions have a shorter and more concise syntax compared to traditional function expressions, which makes it more cleaner and readable.

IIFE:

IIFE stands for immediately invoked function expression . It is a design pattern in javascript where a function is defined and immediately invoked(called) right after the declaration. This pattern is basically used to create a local scope for the variables and avoid polluting the global scope.

lets see the syntax:

// fuction to greet the user 
(function(name) {
    console.log("Hello, " + name + "!");
})("javascript");
//output : Hello, javascript!

It is commonly used to encapsulate the code ,prevent naming conflicts and control variable scope. Before ES6 it was so popular but it decreased
the introduction of block-scoped variables with "let" and "const" in ES6, as they provided a way to create private variables within a function scope. But it is still relevant while working with old Javascript codebase.

This is for today , keep practicing.