library.js is server-side java script, not jQuery. jQuery functionallity can be simulated with eg cheerio
main.js is included in HTML (client-side), here u can use jQuery.
SOLVED JQuery in Widget Template
Use Case - Wanting to use JQuery datepicker in my widget for some reason it wasn't working
NodeBB Version - "1.1.2"'
You should be able to just use the globally provided jQuery object.
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Resolved by using app.loadJQueryUI()