Comparison: Ruby vs JavaScript

Ruby has no var, let or const. You just type the name of the variable and if you want it to be a constant you type it in uppercase.
Ruby uses no parenthesis for functions, unless the function takes arguments, in which case they are optional.
Ruby uses no curly brackets {} to contain the body of a function, it uses the delimiters def and end
if you can’t access something, it just returns null, it doesn’t error
you can not pass functions as arguments as you do in JavaScript
private stuff is actually really 100% private for reals

Ruby has:

no semicolons at the end of every line;
no this keyword
no return, it returns whatever is in the last line (return exists but it used to exit early)

Ruby specific features:

null is called nil
length is also called size and count, so you can use any of them with the same result
three equals (x === y) is just two equals (x == y)
String concatenation is done using double quotes and adding all code inside this brackets: #{}
In Ruby you can not only do something if some condition happens, you can also do something unless some condition happens
You can also loop while some condition is met or until some condition is met.
getters are called “attribute readers”
setters are called “attribute writers”
setter + getter is called “attribute accessor”
boolean methods have to end in question mark ?
mutation methods end in exclamation mark !. These methods change the object they act on in place, rather than returning a new object.