Run npm install after post-merge hook only when package.json changes. Javascript only.

git-helpers.js

const { exec } = require('./helpers')

const changedFile = (filename, partial) => exec(
  `git diff-tree -r --name-only --no-commit-id ORIG_HEAD HEAD`,
  `Something went wrong reading last git-pulled files list.`,
  true
).then((stdout) => stdout.split('\n')
  .some((name) => {
    let match
    if (partial) {
      match = name.endsWith(filename)
    } else {
      match = name === filename
    }
    return match
  })
)

const checkRun = (filename, command) =>
  changedFile(filename)
    .then((changed) => changed && exec(
      command,
      `Something went wrong executing: "${command}"`
    ))

module.exports = {
  changedFile,
  checkRun,
}

helpers.js

const sh = require('shelljs')
const Promise = require('bluebird')

const exec = (code, errorMsg = 'Something went wrong', silent = false) =>
  new Promise((resolve, reject) => {
    sh.exec(code, { silent }, (errorCode, stdout, stderr) => {
      if (errorCode) {
        reject(stderr || errorMsg)
      }
      resolve(stdout)
    })
  })

module.exports = { exec }

post-merge-action.js

#!/usr/bin/env node

const { echo, exit } = require('shelljs')
const { checkRun } = require('./git-utils')

const packageJSON = 'package.json'
const defaultCmd = 'npm install'
const {
  filename = packageJSON,
  cmd = defaultCmd,
} = require('yargs').option('f', {
  alias: 'filename',
  requiresArg: true,
  describe: 'Complete filename with path (relative to git repo) to check.',
  type: 'string'
})
  .option('c', {
    alias: 'cmd',
    requiresArg: true,
    describe: 'Command to be executed if the given file is changed',
    type: 'string'
  }).argv

const action = () => checkRun(filename, cmd)

action()
  .then(() => exit(0))
  .catch((err) => {
    echo(err)
    exit(1)
  })