I strongly urge you to learn the basics of TypeScript.
TypeScript is opt-in; you can make it as strict or as loose as you want. You don’t have to strictly type everything.
Since Ionic is already in TypeScript, you will lose a lot of functionality if you stick to regular JS-- such as autocompletion for property names in your IDE/code editor.
Also, there’s ton’s of good tutorials and books on TypeScript, and the official documentation is a great place to start.