Q:

What is valid syntax of and what are uses of @ at javascript?

Have viewed 
@propertyOrIdentifier // ? What does this mean and do?

used within apparent plain objects or class assignment at Questions and Answers at stackoverflow.
What is the @ symbol or character in javascript? What are valid uses?

A:

It is called a decorator.
https://github.com/wycats/javascript-decorators
Medium - Exploring es7 decorators

