This documentation answers my question very poorly. I didn't understand those explanations. Can someone say in simpler words? Maybe with examples if it's hard to choose simple words?
EDIT also added peerDependencies
, which is closely related and might cause confusion.
Summary of important behavior differences:
dependencies
are installed on both:
npm install
from a directory that contains package.json
npm install $package
on any other directorydevDependencies
are:
npm install
on a directory that contains package.json
, unless you pass the --production
flag (go upvote Gayan Charith's answer).npm install "$package"
on any other directory, unless you give it the --dev
option.npm install
, and you have to solve the dependency yourself manually. When running, if the dependency is missing, you get an error (mentioned by @nextgentech)Transitivity (mentioned by Ben Hutchison):
dependencies
are installed transitively: if A requires B, and B requires C, then C gets installed, otherwise, B could not work, and neither would A.
devDependencies
is not installed transitively. E.g. we don't need to test B to test A, so B's testing dependencies can be left out.
Related options not discussed here:
bundledDependencies
which is discussed on the following question: Advantages of bundledDependencies over normal dependencies in NPMoptionalDependencies
(mentioned by Aidan Feldman)dependencies
are required to run, devDependencies
only to develop, e.g.: unit tests, CoffeeScript to JavaScript transpilation, minification, ...
If you are going to develop a package, you download it (e.g. via git clone
), go to its root which contains package.json
, and run:
npm install
Since you have the actual source, it is clear that you want to develop it, so by default, both dependencies
(since you must, of course, run to develop) and devDependency
dependencies are also installed.
If however, you are only an end user who just wants to install a package to use it, you will do from any directory:
npm install "$package"
In that case, you normally don't want the development dependencies, so you just get what is needed to use the package: dependencies
.
If you really want to install development packages in that case, you can set the dev
configuration option to true
, possibly from the command line as:
npm install "$package" --dev
The option is false
by default since this is a much less common case.
(Tested before 3.0)
Source: https://nodejs.org/en/blog/npm/peer-dependencies/
With regular dependencies, you can have multiple versions of the dependency: it's simply installed inside the node_modules
of the dependency.
E.g. if dependency1
and dependency2
both depend on dependency3
at different versions the project tree will look like:
root/node_modules/
|
+- dependency1/node_modules/
| |
| +- dependency3 v1.0/
|
|
+- dependency2/node_modules/
|
+- dependency3 v2.0/
Plugins, however, are packages that normally don't require the other package, which is called the host in this context. Instead:
E.g. if dependency1
and dependency2
peer depend on dependency3
, the project tree will look like:
root/node_modules/
|
+- dependency1/
|
+- dependency2/
|
+- dependency3 v1.0/
This happens even though you never mention dependency3
in your package.json
file.
I think this is an instance of the Inversion of Control design pattern.
A prototypical example of peer dependencies is Grunt, the host, and its plugins.
For example, on a Grunt plugin like https://github.com/gruntjs/grunt-contrib-uglify, you will see that:
grunt
is a peer-dependency
require('grunt')
is under tests/
: it's not actually used by the program.Then, when the user will use a plugin, he will implicitly require the plugin from the Gruntfile
by adding a grunt.loadNpmTasks('grunt-contrib-uglify')
line, but it's grunt
that the user will call directly.
This would not work then if each plugin required a different Grunt version.
I think the documentation answers the question quite well, maybe you are not just familiar enough with node / other package managers. I probably only understand it because I know a bit about Ruby bundler.
The key line is:
These things will be installed when doing npm link or npm install from the root of a package and can be managed like any other npm configuration parameter. See npm-config(7) for more on the topic.
And then under npm-config(7) find dev
:
Default: false
Type: Boolean
Install dev-dependencies along with packages.