Error Codes:

1. If variable name starts with number below issue

Error- an identifier or keyword can not immediately follow a numeric literal(invalid or unexpected token)

let 1name= "Bal"; 

2.  if reserved keyword(let) is used twice

Error - let is disallowed as a lexically bound name

let let name ="Bal" 

// javascript stops executing if there is an error in earlier line

3. if variable is not define but trying to print/access , throws error - 

Uncaught ReferenceError : fullName is not defined

console.log(fullName) 

4. Error: Uncaught SyntaxError: Unexpected token 

Cause: syntax error in object,variable,function whatever is mentioned after Unexpected token