Alright?
I'm developing a system to check in online, but I need this system to work offline as well since he was born.
For example: I am connected, I loaded all contacts and I wish he armazenasse it cached on the user's device, because the internet is down I can still access this data.
How could I do this?
Hug.
HTML5 LocalStorage is your answer. Literally, in JavaScript, if the localStorage object is not undefined, you can use it to permanently store variables. From there, write a JSON export and use JavaScript to dump all your data into this to re-use later.