install-yarn.js

// do it inline in sync way
// to make it work in non-npm environment
var yarnBin
  , executioner
  , path = require('path')
  , node = process.argv[0]
  ;

if (process.env['npm_execpath'] && process.env['npm_execpath'].match(/[\/\\]yarn(-v\d+\.\d+\.\d+)?[\/\\]bin[\/\\]yarn\.js$/)) {
  var execPath = process.env['npm_execpath'];
  var expectedPath = path.join('bin', 'yarn.js');
  if (execPath.slice(-1 * expectedPath.length) === expectedPath) {
    yarnBin = path.resolve(execPath, '..', '..', 'lib', 'cli');
  }
}

// if no yarn module found, don't expose any function
// to allow upstream modules find alternatives
module.exports = null;

if (yarnBin) {
  executioner = require('executioner');

  module.exports = function(packages, extra, done) {
    var options = {
      node    : node,
      yarn    : yarnBin,
      // escape package names@versions
      packages: packages.map((pkg) => '"' + pkg + '"').join(' ')
    };

    executioner('"${node}" "${yarn}" add --peer --no-lockfile ${packages}', options, function(error, result) {
      if (error) {
        console.error('Unable to install peerDependencies', error);
        process.exit(1);
        return;
      }
      done(result);
    });

    // Looks like yarn shows last line from the output of sub-scripts
    console.log('Installing peerDependencies as devDependencies...');
  };
}