Java Script

Known as 

"camelCase"

"snake_case"

Naming Conventions

A declaration of   let  declares a  set of Variables that can be altered.

A declaration of   const  declares a set of Variables that should not be altered.

var is pre 2016 JS update and was used before the introduction of   let  and  const  

   "camelCase" is a Java Script naming convention.  Capital letters      are used instead of spaces when using multiple-word identiffiers

let  firstNamePerson  =  23;

   "snake_case" is a naming convention used in many progamming

    lauguages.   An underscore is used instead of spaces when using multiple-word identiffiers

let  first_name_person  =  23;

Either convention can be used in Java script, however for clean coding, conventions should not be mixed.