post-checkout.js

#! /usr/bin/env node

/**
 * This file does not use typescript as it's run before you've necessarily run `yarn install`
 * As such, we can only use built-in modules here.
 */

function main() {
  let [
    // Arguments from the githook: https://git-scm.com/docs/githooks#_post_checkout
    prevHead,
    head,
    isFileCheckout
  ] = process.env.HUSKY_GIT_PARAMS.split(' ', 3);

  // we're doing `git checkout foo.js` so don't run:
  if (isFileCheckout === '0') {
    return;
  }

  const diffArgs = [prevHead, head, '--', 'package.json'];
  const args = ['diff', '--exit-code', '--quiet', ...diffArgs];

  const result = require('child_process').spawnSync('git', args, {
    encoding: 'utf8'
  });

  if (result.status === 1) {
    console.log(`
⚠️  package.json was changed! You may wish to run:

  $ yarn install

You can see the changes with:

  $ git diff ${diffArgs.join(' ')}
`);
  }
}

main();