What to consider when testing in javascript web application

I have developed applications without tests before, and it has been fine till now. But now I am actually looking into developing my web applications using Tests, now I am doing tests with Jest. I mostly develop applications with Vue.js/Nuxt.js.

What I am confused about is do you test all your components including the Headers component (Top navigation), Pages (For example index.vue, About.vue), Footer components?

Or just the components which are dynamic, meaning those which can be changed by a user? Like Forms, buttons, inputs? If so, how do you consider which should be tested in general for unit-testing and integration testing?