This is the result to complete the Exercise Tracker project. Instructions for building this project can be found at https://www.freecodecamp.org/learn/apis-and-microservices/apis-and-microservices-projects/exercise-tracker

 * Avoid duplicate username
 * Avoid duplicate user_id using recursive function
 * Using local file to save user input
 * Using id generator to generate user id
 * Using express validator to validate user input

• You should provide your own project, not the example URL.
• You can POST to /api/users with form data username to create a new user.
• The returned response from POST /api/users with form data username will be an object with username and _id properties.
• You can make a GET request to /api/users to get a list of all users.
• The GET request to /api/users returns an array.
• Each element in the array returned from GET /api/users is an object literal containing a user's username and _id.
• You can POST to /api/users/:_id/exercises with form data description, duration, and optionally date. If no date is supplied, the current date will be used.
• The response returned from POST /api/users/:_id/exercises will be the user object with the exercise fields added.
• You can make a GET request to /api/users/:_id/logs to retrieve a full exercise log of any user.
• A request to a user's log GET /api/users/:_id/logs returns a user object with a count property representing the number of exercises that belong to that user.
• A GET request to /api/users/:_id/logs will return the user object with a log array of all the exercises added.
• Each item in the log array that is returned from GET /api/users/:_id/logs is an object that should have a description, duration, and date properties.
• The description property of any object in the log array that is returned from GET /api/users/:_id/logs should be a string.
• The duration property of any object in the log array that is returned from GET /api/users/:_id/logs should be a number.
• The date property of any object in the log array that is returned from GET /api/users/:_id/logs should be a string. Use the dateString format of the Date API.
• You can add from, to and limit parameters to a GET /api/users/:_id/logs request to retrieve part of the log of any user. from and to are dates in yyyy-mm-dd format. limit is an integer of how many logs to send back.