Javascript/30DaysofJS/variablevalidty.yml

# Basics
For a variable that changes at a different time, we use let. If the data does not change at all, we use const. For example, PI, country name, gravity do not change, and we can use const. We will not use var in this challenge and I don't recommend you to use it. It is error prone way of declaring variable it has lots of leak. We will talk more about var, let, and const in detail in other sections (scope). For now, the above explanation is enough.

A valid JavaScript variable name must follow the following rules:

A JavaScript variable name should not begin with a number.
A JavaScript variable name does not allow special characters except dollar sign and underscore.
A JavaScript variable name follows a camelCase convention.
A JavaScript variable name should not have space between words.

### Valid

```
firstName
lastName
country
city
capitalCity
age
isMarried

first_name
last_name
is_married
capital_city

num1
num_1
_num_1
$num1
year2020
year_2020
```

### Invalid
```
first-name
1_num
num_#_1
```