A valid JavaScript identifier shuold must start with a letter,
underscore (_), or dollar sign ($);
subsequent characters can also
be digits (0-9). Because JavaScript is case sensitive,
letters
include the characters "A" through "Z" (uppercase) and the
characters "a" through "z" (lowercase).
Details:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#Variables
