Function Expression vs Arrow Function Expression

// Function Expression
var sayName = function(firstName, lastName) {
    var message = "My name is " + firstName + lastName ;
    console.log(message);
}
// Arrow Function Expression
const sayName = (firstName, lastName) => {
    const message = "My name is " + firstName + lastName;
    console.log(message);
}

Concise Arrow Function Expression

Before

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

After

const add = (a, b) => a + b; //Single line of code
const square = x => x * x;   //Single line of code, single parameter

Function Types Comparison

// Function Declaration
function divide1(a, b) {
  return a / b;
}
// Function Expression
const divide2 = function(a, b) {
  return a / b;
}
// Arrow Function Expression
const divide3 = (a, b) => {
  return a / b;
}
// Arrow Function Expression – Concise 
const divide4 = (a, b) => a / b;