Conditionally build in the postinstall hook without moving your devDependencies to dependencies.

npm install postinstall-build --save

• What does it do?
• Why?
• Usage
  • Options
    • Specifying build dependencies in package.json
• Examples
• Motivation
• Caveats
  • Bugs in Yarn
  • Bugs in npm
  • Excluding source files via .npmignore or files
  • Building a file referenced by package.json bin

1. Check if your build artifacts exist.
2. If not, temporarily install devDependencies and build.
3. Clean up anything left behind… and that’s it!

So that your package with a build step can support Git (and other non-npm) install locations without checking build artifacts into source control or making everyone install your build dependencies. See Motivation for more details.

postinstall-build [options] <artifact> [command]

• --script: Run the given npm script from package.json instead of supplying a full build command. Specified like: --script name or --script=name. This is the recommended way to supply the build command if it is an npm script, because it is guaranteed to use the same $npm_execpath that triggered postinstall (as opposed to potentially using an incompatible version of npm installed in node_modules by a dependency), if the user agent is npm.
• --only-as-dependency: Run only if the package is being installed as a dependency, not if npm install (no args) is being run in the package’s own directory (usually while you are developing the package itself).
• --silent: Silence the build command’s stdout and stderr, as well as any warnings from postinstall-build itself. Fatal errors will still be printed. Note that this may make debugging much more difficult if something goes wrong. Overrides --verbose (the last one specified wins).
• --verbose: Print information about what postinstall-build is doing and why (as well as the usual warnings and errors). Overrides --silent (the last one specified wins).

If neither command nor --script is supplied, the build command defaults to npm run build.

An artifact path is required. It should point to a file or directory that will be generated by the build command. If the file already exists, the build command won’t be run. The build artifact should almost certainly be included in the published npm package, so that normal installs from the npm registry don’t trigger a build (you can build in the prepublish hook, for example). If you want to always build, you may pass a bogus file path, but this is not recommended (if you’re always going to build, just make your devDependencies real dependencies instead of using postinstall-build).

Note that if your command contains arguments (and thus has spaces), you should wrap it in escaped double quotes (\") instead of single quotes for maximum portability – Windows does not treat single-quoted strings as a single parameter. (This is the case in any npm script regardless of postinstall-build usage.)

If you specify a buildDependencies array in package.json, you can control which dependencies are installed before your build command is run. buildDependencies must be an array of package names that also appear in devDependencies. If a package named in buildDependencies does not exist in devDependencies, then it is assumed to already be available (as a global, peer, or production dependency), will not be installed, and a warning will be printed.

Run the build script (the default) if lib doesn’t exist during postinstall:

{
  "scripts": {
    "build": "babel --presets es2015 --out-dir lib src",
    "postinstall": "postinstall-build lib"
  },
  "dependencies": {
    "postinstall-build": "^3.0.0"
  },
  "devDependencies": {
    "babel-cli": "^6.0.0",
    "babel-preset-es2015": "^6.0.0"
  }
}

Run a different script:

{
  "scripts": {
    "build:lib": "babel --presets=es2015 --out-dir=lib src",
    "postinstall": "postinstall-build lib --script build:lib"
  }
}

Run a non-npm script:

{
  "scripts": {
    "postinstall": "postinstall-build dist \"make dist\""
  }
}

Install only the necessary build dependencies:

{
  "scripts": {
    "build": "babel --presets es2015 --out-dir lib src",
    "postinstall": "postinstall-build lib"
  },
  "dependencies": {
    "postinstall-build": "^3.0.0"
  },
  "devDependencies": {
    "ava": "latest",
    "babel-cli": "^6.0.0",
    "babel-preset-es2015": "^6.0.0",
    "nyc": "latest",
    "prettier": "latest"
  },
  "buildDependencies": [
    "babel-cli",
    "babel-preset-es2015"
  ]
}

⚠️ INCORRECT USAGE ⚠️

{
  "scripts": {
    "build": "babel --presets es2015 --out-dir lib src",
    "postinstall": "postinstall-build \"npm run build\""
  }
}

This example is missing a build artifact – or rather, npm run build is mistakenly being passed as the build artifact. Since that file will never exist, the build task is always run. Since npm run build is provided as the build artifact and not the build command, the default build command is used – which happens to also be npm run build. Things will appear to work, but in fact it is building on every postinstall unconditionally. postinstall-build will issue a warning if it suspects the arguments are incorrect.

Sometimes you want to install or depend on a package from someplace other than npm – for example, from a git URL. If the package needs to be transpiled by a tool like Babel, then this can be tricky: most people put their build step in the version or prepublish hooks, and if you’re not installing from npm then this step probably wasn’t run (unless the build artifacts are checked into source control).

One solution is to add a check to the package’s postinstall hook: if the build artifacts don’t exist, then build! The annoying part is that this necessitates having your build dependencies (like Babel or webpack) available – in other words, they’d need to be production dependencies instead of devDependencies, even though the module itself doesn’t require them (unlike real dependencies, they’re only used in the build step). That means even everyone installing from npm wastes time installing them, even though they already have the build artifacts!

This helper fixes that. Just tell it where a build artifact is and what your build step is, and it’ll do the rest. Used as intended, postinstall-build should be in dependencies.

• 'your-package' is not in the npm registry.

  Yarn will read your custom registry setting from .npmrc, but fails to communicate this via the $npm_config_registry environment variable. So any npm commands that were triggered by a Yarn install (like those run by postinstall-build) pick up Yarn‘s default $npm_config_registry setting instead of the one specified in .npmrc.

  For the time being you can solve this by adding a .yarnrc file alongside your .npmrc, which will cause $npm_config_registry to behave as expected.

I recommend using npm 3 or better, except for npm 4.1.x–4.5.x.

There are several distinct bugs in npm itself that you may encounter when using postinstall-build with npm 2. I have not been able to work around these nor even reproduce them locally; they are especially prevalent on the combination of Node 0.12, npm 2, and the Docker environment used by Travis. To the best of my knowledge they are no fault of this package and are widely reported npm bugs.

• extraneous packages

  The prune command is broken in npm 4.1.x–4.5.x, and is unable to correctly prune devDependencies. Thus, when postinstall-build is finishing up, it leaves behind extraneous packages. (See issues #15727, #15669, #15646.)

• postinstall-build: not found

  Sometimes npm triggers postinstall when a package’s dependencies aren’t actually available yet.

• Callback called more than once.

  npm has some faulty async code. This message comes from within the npm codebase and does not refer to any callbacks within postinstall-build.

• ENOENT during npm prune

  npm is probably trying to prune a file that was already removed or never existed. Seems to happen when there is a larger devDependency tree to prune.

• ECONNRESET

  npm has trouble making lots of connections to its own registry. You can use npm config set fetch-retries 5 (for example) to work around this; using the non-HTTPS registry might also help.

When npm installs from a Git repository or any other non-package location, it will first prepare the directory as if it were publishing a package. This includes respecting the .npmignore file and files field in package.json, which means that postinstall scripts may be executed with a subset of the files you need to run your build step. Thus, in order for postinstall-build to work, you should not ignore the source files or any necessary configuration (for example, .babelrc).

This is not ideal, but it’s how npm works. If you are determined to exclude unnecessary source and configuration files from the published npm package, you may want to consider a publishing step that alters the .npmignore or files settings.

If your package.json file uses the bin field, and any of the referenced files do not exist before building, you may see an error like this:

ENOENT: no such file or directory, chmod '[…]/lib/index.js'

This happens because npm needs to symlink any files referenced in the bin field and make them executable, but this step is performed before postinstall. postinstall-build can’t do anything to address this shortcoming, but there is an easy workaround. Create a simple non-built file (that is, not created during the build step) that imports the built file you actually want to target. For example, you could create a top-level file called cli.js like so:

require('./lib/index');

Or, export the program’s behavior in a function and call it:

require('./lib/index').run();

Make sure to update your bin field to the new file (in this case, cli.js) and include it in your npm package and repository.