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.