Plugins declared in config.js do not work

[Buydens]

• I confirm that this is an issue rather than a question.

Bug report

Running yarn dev or yarn build will throw a warning:
warning An error was encounted in plugin "@vuepress/plugin-back-to-top"

When running dev server or after building, none of the plugins work.
And neither of both work.

Steps to reproduce

I simply ran yarn create vuepress and selected the Doc theme preset.

What is expected?

For the plugins to work when added via Yarn and added in the config.js

What is actually happening?

There are two plugins added in config.js:
plugins: [ '@vuepress/plugin-back-to-top', '@vuepress/plugin-medium-zoom', ]

None of these work, both have the same warning when running yarn dev or yarn build.

Other relevant information

I used to run my Vuepress but kept getting these errors, then I decided to use this boilerplate but the issues continue.

Pretty novice in Vuepress but I just followed the documentation and I can't find a way to see what activates these warning.

[Buydens]

It works after running: yarn add -D @vuepress/plugin-back-to-top looks like it just didn't work right out of the box via yarn create vuepress and then going for the docs boilerplate. (for me)

[renqi1996]

I used npm to create my Vuepress project, and I found this problem too. Still do not know how to fix it...

[haoranpb]

@renqi1996 As warned in the document site, try to yarn instead of npm. And as @Buydens suggested, run yarn add -D @vuepress/plugin-back-to-top to add the dependency. If the problem is still there, can you provide more context?

We currently recommend using Yarn instead of npm when installing VuePress into an existing project that has webpack 3.x as a dependency, because npm fails to generate the correct dependency tree in this case.