Install a package

npm install (with no args in a package dir)
npm install <tarball file>
npm install <tarball url>
npm install <folder>
npm install [@<scope>/]<name> [--save|--save-dev|--save-optional] [--save-exact]
npm install [@<scope>/]<name>@<tag>
npm install [@<scope>/]<name>@<version>
npm install [@<scope>/]<name>@<version range>
npm i (with any of the previous argument usage)

This command installs a package, and any packages that it depends on. If the package has a shrinkwrap file, the installation of dependencies will be driven by that. See npm-shrinkwrap(1).

A package is:

Even if you never publish your package, you can still get a lot of benefits of using npm if you just want to write a node program (a), and perhaps if you also want to be able to easily install it elsewhere after packing it up into a tarball (b).

You may combine multiple arguments, and even multiple types of arguments. For example:

npm install sax@">=0.1.0 <0.2.0" bench supervisor

The --tag argument will apply to all of the specified install targets. If a tag with the given name exists, the tagged version is preferred over newer versions.

The --force argument will force npm to fetch remote resources even if a local copy exists on disk.

npm install sax --force

The --global argument will cause npm to install the package globally rather than locally. See npm-folders(5).

The --link argument will cause npm to link global installs into the local space in some cases.

The --no-bin-links argument will prevent npm from creating symlinks for any binaries the package might contain.

The --no-optional argument will prevent optional dependencies from being installed.

The --no-shrinkwrap argument, which will ignore an available shrinkwrap file and use the package.json instead.

The --nodedir=/path/to/node/source argument will allow npm to find the node source code so that npm can compile native modules.

See npm-config(7). Many of the configuration params have some effect on installation, since that's most of what npm does.

To install a package, npm uses the following algorithm:

install(where, what, family, ancestors)
fetch what, unpack to <where>/node_modules/<what>
for each dep in what.dependencies
  resolve dep to precise version
for each dep@version in what.dependencies
    not in <where>/node_modules/<what>/node_modules/*
    and not in <family>
  add precise version deps to <family>
  install(<where>/node_modules/<what>, dep, family)

For this package{dep} structure: A{B,C}, B{C}, C{D}, this algorithm produces:

+-- B
`-- C
    `-- D

That is, the dependency from B to C is satisfied by the fact that A already caused C to be installed at a higher level.

See npm-folders(5) for a more detailed description of the specific folder structures that npm creates.

There are some very rare and pathological edge-cases where a cycle can cause npm to try to install a never-ending tree of packages. Here is the simplest case:

A -> B -> A' -> B' -> A -> B -> A' -> B' -> A -> ...

where A is some version of a package, and A' is a different version of the same package. Because B depends on a different version of A than the one that is already in the tree, it must install a separate copy. The same is true of A', which must install B'. Because B' depends on the original version of A, which has been overridden, the cycle falls into infinite regress.

To avoid this situation, npm flat-out refuses to install any name@version that is already present anywhere in the tree of package folder ancestors. A more correct, but more complex, solution would be to symlink the existing version into the new location. If this ever affects a real use-case, it will be investigated.

Last modified August 20, 2015           Found a typo? Send a pull request!

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