flatn 

flat node dependencies (flatn) is a nodejs package organizer that supports flat directory structures for nodejs package dependencies. flatn was created to allow npm packages being used together with SystemJS and lively.modules while maintaining compatibility with node.js modules at the same time.

flatn installs packages into one or multiple directories and tells nodejs how to resolve packages in there so normal require(...) statements work.

- flat node_modules, actually you can give that directory a custom name
- makes developing multiple inter-dependent packages easy (no symlinking, junction-pointing)
- more efficient package storage
- simple package lookup, friendlier to non-nodejs module systems (like SystemJS)


How it works

Instead of finding packages / modules by convention, with flatn you specify explicitly what package locations should be used for lookup. There are three categories for package locations:

1. collection package directories
2. package directories
3. dev package directories

Collection package dirs specify parent directories in which a subfolder structure points to actual packages, e.g.

``
collection dir
├── acorn
│   ├── 1.2.2
│   │   ├── package.json
│   │   └── src
│   │   └── ...
│   ├── 4.0.13
│   │   ├── package.json
│   │   └── src
│   │   └── ...
├── acorn-es7-plugin
│   └── 1.1.7
│   ├── package.json
│   └── ...
├── acorn-jsx
│   └── 4.0.1
│   └── ...
`

The folder structure will be established by calling flatn install. There can be multiple collection package directories, at runtime they will be all used for package lookup, prioritized by the order in which they where specified.

Package directories denote a patch to individual packages, no special directory structure is necessary. The versioning information will be read from a package.json file that is expected to be at the root of the package directory.

Dev package directories are similar to package directories except that packages specified there will have the highest priority during lookup. For packages of collection or normal package dirs, the version will be used during looking, according to semver. I.e. if you have a package bar that dependes on acorn@^1 then the collection dir/acorn/1.2.2 location will be found using the example from above. If you specify acorn as a dev package, however, versioning info when requiring this package will be disregarded. This makes sure that all packages that depend on a dev package will use only a single version, so you can be sure that changes to the dev package will have an effect on all dependends.

Collection/ package/ dev package directories can be specified via environment variables, comamnd line or modified at runtime in JavaScript.

Rationale

npm installs package dependencies into node_modules folder inside of packages that require them. The internal module resolve mechanism in nodejs then traverses the file system tree upward when it wants to load (require) one module from another one, looking into the node_modules folder to the matching package.

This has a number of negative consequences:
1. __During development__: When you develop multiple packages that depend on each other you typically don't want them to be installed like normal dependencies but rather link the packages to each other. npm link allows to do this but installs the links globally, so you cannot use multiple versions of you packages under development at the same time.

2. __Performance__: In the current npm version 4, linked packages (either symlinked by hand or via npm link) sometimes seem to lead to very slow install / update operations, sometimes freezing the install / update process completely.

3. __Sharing__: Local packages that depend on the same dependencies each install a seperate version of them, even when npm linked. Depending on how many packages you develop this can waste considerable disk space (gigabytes) and considerably slows down install / update processes.

4. __Simplicity of module resolution algorithm and usage with other module systems__: Since npm version 3, npm tries to partially flatten out the dependency tree to avoid redundancies. However, this has now the drawback that you cannot rely on the package.json dependency fields to find packages anymore deeper down in the tree. You basically need to resolve packages via nodejs or use your own implementation of the npm lookup algorithm if you want to find packages with a different module system. This complicates using node_modules especially in the browser context and with EcmaScript modules and System loaders such as SystemJS.

$3

1. __Custom package directories__: To solve these problems, flattn can install packages in one or multiple shared directories that differ from the normal node_modules folders. This directory / directories can either be local to your own packages or reside somewhere else on your file system.

2. __Directories can be shared but are not global__ By specifying which directories to use via environment variables or command line arguments, installed packages can be shared by multiple local packages. This allows to minimize the number of installed dependencies.

3. __Directory structure is flat and straightforward__: No nested package structures are necessary, packages are installed via name@version inside the specified directories, github dependencies are supported. Finding packages is done by comparing the version of the package with the version requirement of the callsite. _Versions of package dependencies are still resolved correctly, i.e. flatn supports the case that packages might require the same dependency in multiple versions._

Unlike yarn install --flat multiple versions of the same dependency can coexist.

By importing module-resolver.js into your nodejs process and specifying the package directories via environment variables, nodejs will then use those directories when it tries to load modules. Alternatively to manually importing module-resolver.js you can use bin/node to start nodejs.

$3

#### npm

The package install strategy of npm version 1 and 2 was to install all dependencies of a package it listed in package.json into a node_modules folder in the packages base directory. Sub-packages were installed in the same way recursively. The downside of this approach is that for lots of dependencies the node_modules folder structure can grow very large and very deep.

npm version >= 3 improves that by using a deduplication and localization strategy. This alleviates the deep folder problem but still keeps a the node_modules tree structure around. The downside of this approach is that the location of particular packages is not transparent anymore. In order to find a package in the dependency tree one has to essentially execute the node.js module lookup algorithm. This becomes a problem when using packages and their dependencies in the browser or with other module systems (like SystemJS or lively.modules).

Also, sharing of installed packages across local projects is not possible. If you have mutliple packages you develop and that are therefore not installed via npm, dependencies of those are not shared, each gets its own node_modules tree. Depending on how many local projects you have this can waste gigabytes.

Furthermore, local packages do not know about each other. If you have a packaga-a that requires a package-b you either have to place package-b directly inside the node_modules of package-a or npm link package-b or symlink package-b => package-a. npm link is global so you cannot have multiple versions of package-b. As of npm version 5, symlinking freezes npm update and npm install processes or leads to endless recursive calls (which sometimes also happens with npm link)...

The name and location of the node_module folder cannot be customized.

#### yarn

The yarn package manager is an alternative to npm but follows the same directory organization.

Yarn has an option to install flat dependencies. However, in the case of version conflicts, the user has to specify a resolution that picks one of the conflicting versions. This can potentially lead to runtime issues as packages have to use the wrong version of their dependencies.

Yarn 1.0 added a feature called workspaces: Similarly to lerna (see below) it avoids installing the same dependencies for hierarchically organized packages, specifically packages extracted from monorepos. Unlike flatn, this requires the packages under development to be layed out hierarchically. This is reasonable for splitting up functionality across sub-packages but requires at least one "entry" package. Workspaces then create node_modules folders similarly to the Lerna approach. In comparison, flatn allows to loosely combine packages under development even if they have no "hierarchical relationship".

#### Lerna

Lerna is a tool for managing multi-package mono repositories and builds on top of npm and yarn. Lerna can optionally reduce the file system overhead of package dependencies by hoisting dependencies when generating directories for the multiple packages extracted from the mono repo. Hoisting occurs using node_modules folders and requires packages to be nested into each other. flatn in comparison works well without requiring development packages to be structured in a certain way and can place the dependencies anywhere in the file system (i.e. a folder outside of the development package structure).

#### flatn

flatn uses a different strategy by allowing a fully custom location or multiple locations for package dependencies. Those dependencies can then be shared by local packages. No symlinking happens but the node.js runtime is extended to lookup the dependencies in the right location. Additionally, flatn provides an option to specify development packages that are then made known to the runtime similarly and that are not constrained by their version specifiers.

$3

Let's say we have a local package foo with package.json

`json
{
"name": "foo",
"version": "0.1.0",
"dependencies": {
"chalk": "^1"
}
}
`

#### Install + require

We want to use a custom folder inside the projects directory to store the dependencies. By running
`sh
$ mkdir deps
$ flatn --packages deps install
`

we install the "chalk" dependency inside the deps folder. We can now set the FLATN_PACKAGE_COLLECTION_DIRS environment variable and run nodejs:
`sh
$ eval $( flatn --packages deps env )
$ node -p 'require("chalk").blue.bgRed.bold("it works!!!");'
``

#### Using local packages without install

Let's say we have another module bar that is local and that we want to use from foo:

`sh
$ eval $( flatn --packages deps --dev-package /path/to/foo env )
$ node -p 'require("foo").doSomething(here)'
``

Command line usage

npm install -g flatn then

`
flatn – flat node dependencies

Usage: flatn [generic args] command [command args]

Generic args:
--packages / -C Specifies a directory whose subdirectories are expected to be all packages ("package collection" dir).
The equivalent environment variable is FLATN_PACKAGE_COLLECTION_DIRS.
--dev-package / -D Specifies a development package. Dev packages will always
be built and will override all packages with the same name. When a module
requires the name of a dev package, the package will always match, no matter
its version.
The equivalent environment variable is FLATN_DEV_PACKAGE_DIRS.
--package/ -P Specifies the path to a single package.
The equivalent environment variable is FLATN_PACKAGE_DIRS.

(Repeat -C/-D/-P multiple times to specify any number of directories.)

Commands:
help Print this help
list List all packages that can be reached via the flatn package directories
specified in the environment and via generic arguments.
install Usage without name: Downloads dependencies of the package in the
current directory and runs build tasks (with --save and --save-dev) also adds
to package.json in current dir
install name Installs the package name in the first collection package dir
specified. With arguments --save or --save-dev it adds this package to the
dependencies or devDepedencies entry of the current package's package.json
node Starts a new nodejs process that resolves modules usin the specified
package directories. To path arguments to nodejs use "--" followed by any
normal nodejs argument(s).
env Print the environment variables of the current configuration. Optional argument: --json

Environment:
Use the environment variables
- FLATN_PACKAGE_COLLECTION_DIRS
- FLATN_PACKAGE_DIRS
- FLATN_DEV_PACKAGE_DIRS
to specify package directories. The variables correspond to the -C, -P, and -D
generic arguments. Use ":" to specify multiple directories, e.g.
FLATN_DEV_PACKAGE_DIRS=/home/user/package1:/home/user/package2.
Note: All directories have to be absolute.
``