https://thomashunter.name/blog/the-long-road-to-asyncawait-in-javascript/

stage-0.js

var level_result = createLevel(12, {data: true});
console.log(level_result);

function createLevel(user_id, level_data) {
  var user = getUser(user_id);
  var can_create = canCreate(user);

  if (!can_create) {
    return null;
  }

  var level = saveLevel(user, level_data);

  return level;
}

function getUser(user_id) {
  return {
    id: user_id,
    nickname: 'tlhunter'
  };
}

function canCreate(user) {
  return user.id === 12;
}

function saveLevel(user, data) {
  return {
    id: 100,
    owner: user.nickname,
    data: data
  };
}

stage-1.js

createLevel(12, {data: true}, function(level_result) {
  console.log(level_result);
});

function createLevel(user_id, level_data, cb) {
  getUser(user_id, function(user) {
    canCreate(user, function(can_create) {
      if (!can_create) {
        return cb(null);
      }
      saveLevel(user, level_data, function(level) {
        cb(level);
      });
    });
  });
}

function getUser(user_id, cb) {
  setTimeout(function() {
    cb({
      id: user_id,
      nickname: 'tlhunter'
    });
  }, 100);
}

function canCreate(user, cb) {
  setTimeout(function() {
    cb(user.id === 12);
  }, 100);
}

function saveLevel(user, data, cb) {
  setTimeout(function() {
    cb({
      id: 100,
      owner: user.nickname,
      data: data
    });
  }, 100);
}

stage-2.js

createLevel(12, {data: true}).then(function(level_result) {
  console.log(level_result);
});

function createLevel(user_id, level_data, cb) {
  var user = null;
  return getUser(user_id).then(function(_user) {
    user = _user;
    return canCreate(_user);
  }).then(function(can_create) {
    if (!can_create) {
      return null;
    }

    return saveLevel(user, level_data);
  });
}

function getUser(user_id) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: user_id,
        nickname: 'tlhunter'
      });
    }, 100);
  });
}

function canCreate(user) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve(user.id === 12);
    }, 100);
  });
}

function saveLevel(user, data) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: 100,
        owner: user.nickname,
        data: data
      });
    }, 100);
  });
}

stage-3.5.js

/* jshint esnext: true */
var co = require('co');

createLevel(12, {data: true}).then(function(level_data) {
  console.log(level_data);
});

function createLevel(user_id, level_data) {
  return co(function * createLevel() {
    var user = yield getUser(user_id);
    var can_create = yield canCreate(user);

    if (!can_create) {
      return null;
    }

    var level = yield saveLevel(user, level_data);

    return level;
  });
}

function getUser(user_id) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: user_id,
        nickname: 'tlhunter'
      });
    }, 100);
  });
}

function canCreate(user) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve(user.id === 12);
    }, 100);
  });
}

function saveLevel(user, data) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: 100,
        owner: user.nickname,
        data: data
      });
    }, 100);
  });
}

stage-3.js

/* jshint esnext: true */
var generator = createLevel(12, {data: true});

generator.next().value.then(function(user) {
   return generator.next(user).value.then(function(can_create) {
     return generator.next(can_create).value.then(function(level_result) {
       console.log(level_result);
     });
   });
 });

function * createLevel(user_id, level_data) {
  var user = yield getUser(user_id);
  var can_create = yield canCreate(user);

  if (!can_create) {
    return null;
  }

  var level = yield saveLevel(user, level_data);

  return level;
}

function getUser(user_id) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: user_id,
        nickname: 'tlhunter'
      });
    }, 100);
  });
}

function canCreate(user) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve(user.id === 12);
    }, 100);
  });
}

function saveLevel(user, data) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: 100,
        owner: user.nickname,
        data: data
      });
    }, 100);
  });
}

stage-4.es5.js

'use strict';

var _regeneratorRuntime = require('babel-runtime/regenerator')['default'];

var _Promise = require('babel-runtime/core-js/promise')['default'];

createLevel(12, { data: true }).then(function (level_data) {
  console.log(level_data);
});

function createLevel(user_id, level_data) {
  var user, can_create, level;
  return _regeneratorRuntime.async(function createLevel$(context$1$0) {
    while (1) switch (context$1$0.prev = context$1$0.next) {
      case 0:
        context$1$0.next = 2;
        return _regeneratorRuntime.awrap(getUser(user_id));

      case 2:
        user = context$1$0.sent;
        context$1$0.next = 5;
        return _regeneratorRuntime.awrap(canCreate(user));

      case 5:
        can_create = context$1$0.sent;

        if (can_create) {
          context$1$0.next = 8;
          break;
        }

        return context$1$0.abrupt('return', null);

      case 8:
        context$1$0.next = 10;
        return _regeneratorRuntime.awrap(saveLevel(user, level_data));

      case 10:
        level = context$1$0.sent;
        return context$1$0.abrupt('return', level);

      case 12:
      case 'end':
        return context$1$0.stop();
    }
  }, null, this);
}

function getUser(user_id) {
  return new _Promise(function (resolve) {
    setTimeout(function () {
      resolve({
        id: user_id,
        nickname: 'tlhunter'
      });
    }, 100);
  });
}

function canCreate(user) {
  return new _Promise(function (resolve) {
    setTimeout(function () {
      resolve(user.id === 12);
    }, 100);
  });
}

function saveLevel(user, data) {
  return new _Promise(function (resolve) {
    setTimeout(function () {
      resolve({
        id: 100,
        owner: user.nickname,
        data: data
      });
    }, 100);
  });
}

stage-4.js

createLevel(12, {data: true}).then(function(level_data) {
  console.log(level_data);
});

async function createLevel(user_id, level_data) {
  var user = await getUser(user_id);
  var can_create = await canCreate(user);

  if (!can_create) {
    return null;
  }

  var level = await saveLevel(user, level_data);

  return level;
}

function getUser(user_id) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: user_id,
        nickname: 'tlhunter'
      });
    }, 100);
  });
}

function canCreate(user) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve(user.id === 12);
    }, 100);
  });
}

function saveLevel(user, data) {
  return new Promise(function(resolve) {
    setTimeout(function() {
      resolve({
        id: 100,
        owner: user.nickname,
        data: data
      });
    }, 100);
  });
}