How to use the shelljs.which function in shelljs

var promise = new Promise(function (resolve, reject) {
        if (require('shelljs').which(browser)) {
            return resolve();
        } else {
            var regQPre = 'reg QUERY "HKLM\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\App Paths\\';
            var regQPost = '.EXE" /v ""';
            var regQuery = regQPre + browser.split(' ')[0] + regQPost;

            child_process.exec(regQuery, function (err, stdout, stderr) {
                if (err) {
                    // The registry key does not exist, which just means the app is not installed.
                    reject(err);
                } else {
                    var result = regItemPattern.exec(stdout);
                    if (fs.existsSync(trimRegPath(result[2]))) {
                        resolve();
                    } else {
                        // The default value is not a file that exists, which means the app is not installed.

// after this check the path with the non standard commands
    let cPath = null;
    _.forEach(definition.otherCmds, (entry) => {
      const tryPath = path.resolve(cmdPath, entry);
      if (shelljs.which(tryPath)) {
        cPath = tryPath;
      }
    });
    if (cPath) {
      return cPath;
    }
  }


  // after this check just the standard command
  if (shelljs.which(definition.standardCmd)) {
    return definition.standardCmd;
  }
  // after this check the non standard commands
  let otherCmd = null;
  _.forEach(definition.otherCmds, (entry) => {
    if (shelljs.which(entry)) {
      otherCmd = entry;
    }
  });
  if (otherCmd) {
    return otherCmd;
  }
  // if none of the commands work return false
  return false;
}
/**

// Install packages
    if (ctx.answers.pm === 'yarn') {
      ctx.yarnInstall();
    } else {
      ctx.npmInstall();
    }

    // Format code with xo
    await execa(`./node_modules/.bin/xo`, ['--fix'], {
      cwd: ctx.folderPath,
      stdio: 'inherit'
    });

    // Run tests
    await execa(which(ctx.answers.pm).stdout, ['test'], {
      cwd: ctx.folderPath,
      stdio: 'inherit'
    });

    ctx.showTip();
  }
};

deps.ruby.versionCheck = deps.ruby.exe + ' -v' + deps.ruby.exeSuffix;
            deps.gem.exe = fs.readFileSync(runtime.binDir + '/gem', 'utf-8').trim().replace(' $*', '');
            deps.gem.writeShim = false;
            deps.bundler.exe = runtime.binDir + '/bundler'; // <-- not a lanyon shim, it's a real gem bin
            deps.bundler.writeShim = false;
            _context.next = 62;
            break;

          case 23:
            if (!utils.satisfied(runtime, 'ruby', undefined, 'system')) {
              _context.next = 29;
              break;
            }

            rubyProvider = 'system';
            deps.gem.exe = shell.which('gem').stdout;
            deps.bundler.exe = shell.which('bundler').stdout;
            _context.next = 62;
            break;

          case 29:
            if (!utils.satisfied(runtime, 'docker')) {
              _context.next = 42;
              break;
            }

            rubyProvider = 'docker';

            if (!(process.env.DOCKER_BUILD === '1')) {
              _context.next = 37;
              break;
            }

gulp.task('git-check', function(done) {
  if (!sh.which('git')) {
    console.log(
      '  ' + gutil.colors.red('Git is not installed.'),
      '\n  Git, the version control system, is required to download Ionic.',
      '\n  Download git here:', gutil.colors.cyan('http://git-scm.com/downloads') + '.',
      '\n  Once git is installed, run \'' + gutil.colors.cyan('gulp install') + '\' again.'
    );
    process.exit(1);
  }
  done();
});

export function isCLIInstalled(): boolean {
  let isInstalled = false;
  try {
    if (which('sfdx')) {
      isInstalled = true;
    }
  } catch (e) {
    console.error('An error happened while looking for sfdx cli', e);
  }
  return isInstalled;
}

compile_vyper(contractFiles, _options, cb) {
    const self = this;
    if (!contractFiles || !contractFiles.length) {
      return cb();
    }

    const vyper = shelljs.which('vyper');
    if (!vyper) {
      self.logger.warn(__('%s is not installed on your machine', 'Vyper'));
      self.logger.info(__('You can install it by visiting: %s', 'https://vyper.readthedocs.io/en/latest/installing-vyper.html'));
      return cb();
    }
    self.logger.info(__("compiling Vyper contracts") + "...");

    const compiled_object = {};
    async.each(contractFiles,
      function (file, fileCb) {
        const className = path.basename(file.path).split('.')[0];
        compiled_object[className] = {};
        async.parallel([
          function getByteCode(paraCb) {
            self.compileVyperContract(file.path, false, (err, byteCode) => {
              if (err) {

const checkYo = () => {
  if (!Shell.which('yo')) {
    console.log(colors.red('This command requires yo to be installed.'))
    console.log(colors.green('Installing yo...'))
    Shell.exec('CI=true npm i -g yo')
  }
}

function forgivingWhichSync(cmd) {
    try {
        return fs.realpathSync(shelljs.which(cmd));
    } catch (e) {
        return '';
    }
}

function forgivingWhichSync (cmd) {
    try {
        return fs.realpathSync(shelljs.which(cmd));
    } catch (e) {
        return '';
    }
}