What Is Asynchronous Control Flow?
Let variables and arrow functions are ES6 features. Older browsers still in use, such as Internet Explorer 11 and below, or Safari 7 and below, do not support these features. Check out http://kangax.github.io/es5-compat-table/es6/ for the current compatibility charts.
You want a better understanding of asynchronous control flow.
Asynchronous control flow is the name of a set of patterns to help control the order of execution of asynchronous code. To manage ...