To write simple way as “Arrow Function”

2.17 Arrow function, the basics - link

// arrow function
let func = (arg1, arg2, ..., argN) => expression;

// example
let sum = (a, b) => a + b;
/* This arrow function is a shorter form of:

let sum = function(a, b) {
	return a + b;
};
*/

alert( sum(1, 2) ); // 3

• only on argument, parentheses can be omitted

let double = n => n * 2;

alert( double(3) );

• it must be presented parentheses when there are no arguments.

let sayHi = () => alert("Hello");

sayHi();

// to dynamically create a function
let age = prompt();

let welcome = (age < 18) ?
	() => alert() :
	() => alert();
	
welcome();

Mutiline arrow functions

// mutiple expressions and statements

let sum = (a, b) => {  
  let result = a + b;
  // if we use curly braces, then we need an explicit "return"
  return result; 
};

alert( sum(1, 2) ); // 3