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Write a function (without using external libraries) in ES6 that converts the user entered date formatted as
M/D/YYYY to the format
YYYYMMDD which is required by an API endpoint. It should convert
”20161231” for example. Furthermore, it should validate that the passed date is formatted correctly using a Regular Expression (let's assume all months have 31 days). Write unit tests for your convert function using the syntax of a BDD-style test framework of your choice.