node.js and npm v3: How to add Travis tests compatible peerDependencies in package.json

Question

With peerDependencies in package.json I can make sure that the user has a specific module in the application rootfolder:

module_a

var modB = require('module_b')
...

package.json of module_a

...
"peerDependencies": {
  "module_b": "^1.0.1"
},
"dependencies": {
},
...

my_app

var modA = require('module_a')
var modB = require('module_b')
...

file structure

With npm v1/v2 this configuration works perfect: npm install module_a installs module_a and module_b in the rootfolder:

my_app
  node_modules
    module_a
    module_b

Great, that is what I want!

npm v3

But while installing with npm install module_a, npm v2 prints this warning:

npm WARN peerDependencies The peer dependency module_b@^1.0.1 included
from module_a will no longer be automatically installed to fulfill the
peerDependency in npm 3+. Your application will need to depend on it
explicitly.

So, npm v3 will not install the peer dependency automatically, I have to install it manually for my_app to achieve the same result.

The problem

But what about in my tests when using npm v3?

npm v3 doesn't install the peer dependency, so travis will fail because it cannot find module_b. But I cannot add the module as regular dependency because than my_app and module_a use different "instances" of module_b:

my_app
  node_modules
    module_a
      node_modules
        module_b         // used by module_a
    module_b             // used by my_app

That is not what I want because module_a changes some parameters in module_b, but this changes aren't visible in my_app.

Question

How can I add module_b as (peer) dependency in the rootfolder without breaking the travis tests from module_a?

Thank you.

Answer 1

If a module is not in the dependencies list but is required to run the tests, then you should list it as a devDependency (regardless of whether it's also a peerDependency).

Answer 2

I solved this by installing the peer dependencies in travis' before_install hook.
My .travis.yml looks like so:

language: node_js
node_js:
  - "6.1"
  - "5.11"
  - "4.4"
before_install:
  - "npm install peerDependency"