Which keywords can be used for the declaration of variables or constants? 
// variable declaration
let a = 1;

// constant declaration
const C = 3;

// legacy variable declaration (not recommended any longer)
var b = 2;
How to determine the type of a variable? 
typeof aVariable;
Three ways to define a string. 
let doubleQuotes = "string with double quotes";

let singleQuotes = 'string with single quotes';

There is no difference in behavior between double and single quotes. Double quotes are more Java-like, whereas single quotes are handy when used directly inside HTML code. 

let backtick = `You can use either
 "${doubleQuotes}"
 or
 '${singleQuotes}'
`;

Backtick notation allows for embedded expressions, and implicit line breaks (without "\n\").

How to define numbers? 
// integers and floats are both stored with the data type "number":
let x = 42;
let y = 42.3;

// for big integers, use the "n" suffix:
let z = 84n
What is the difference between "undefined" and "null"? 
let x;
// at this point, x is still undefined:
console.log(x);

// x becomes null when explicitly assigning it:
x = null;
console.log(x);
How to create an object using literal notation? 
let x = { a: 1 };
How to create an array object using literal notation? 
let x = [1, 2, 3];
How to create a regular expression object using literal notation? 
let x = /ab+c/;