how to Export html form data to json file

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Javascript

how to save html registration form data into json file then login based on it
without using any server side scripting languages.
just with javascript, jquery

html file "simple form" contains:
First Name, Middle Name, Last Name, User Name, Password, Confirm Password , E-Mail, Phone

i want when user enters his data and click register button, his data will be saved in json file

surely on the server

i have created this json file on the server before "C:\inetpub\wwwroot\usersData.txt"

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and based on this file "usersData.txt", when another user want to register again, i need to check in this json file "usersData.txt" that user name is unique, if it is a unique user name, save his data in the json file and vice versa
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finally, i want to how to update the password in this json file when i use change_password() JS function

please i need the help quickly
Thanks v.much in advance