It is very common to use anonymous functions as parameters in JavaScript. Event binding, async callback and so on all use anonymous function. ES6 introduced new Arrow Functions which has shorter syntax as a replacement for anonymous function. The basic syntax is: (arguments) => {expression or return value}.
ECMAScript 2015 (ES6) provides a stricter way to declare variables: let which can be used to supersede legacy var statement. The let statement declares a block scope local variable while the variables declared with var are scoped to the function block level. Although it is very loose and comfortable to declare variables with var, but this characteristic usually leads to some unexpected behaviors, especially in a complicated project.