Sketchpad: Event-driven design in TypeScript

typed-event-driven.ts

// Playing around with TypeScript
// Inspiration: https://github.com/abdullin/omni

// core.event
interface DomainEventType {
  name: string;
}

interface DomainEvent {
  _type: DomainEventType;
}

// lang.ids
type EventId = string;
type TaskId = string;

// lang.events
var TaskAdded: DomainEventType = {name: 'TaskAdded'};
interface TaskAddedAttributes {
  eventId: EventId;
  taskId: TaskId;
  name: string;
}
interface TaskAddedEvent extends DomainEvent, TaskAddedAttributes {}
function newTaskAdded(attr: TaskAddedAttributes): TaskAddedEvent {
  return objectAssign(attr, {_type: TaskAdded});
}

var TaskRemoved: DomainEventType = {name: 'TaskRemoved'};
interface TaskRemovedAttributes {
  eventId: EventId;
  taskId: TaskId;
}
interface TaskRemovedEvent extends DomainEvent, TaskRemovedAttributes {}
function newTaskRemoved(attr: TaskRemovedAttributes): TaskRemovedEvent {
  return objectAssign(attr, {_type: TaskRemoved});
}

// views.store
interface TaskItem {
  taskId: TaskId;
  name: string;
  inbox: boolean;
}
interface Store {
  [index: string]: TaskItem;
}

function addTask(s: Store, id: TaskId, name: string): Store {
  s[id] = {
    taskId: id,
    name: name,
    inbox: false
  };
  return s;
}

function removeTask(s: Store, id: TaskId): Store {
  delete s[id];
  return s;
}

// views.denormalizer
function handleEvent(s: Store, e: any): Store {
  // Would've liked to use `e: DomainEvent` in func signature but can't...
  switch (e._type) {
    case TaskAdded:
      s = addTask(s, e.taskId, e.name);
      break;
    case TaskRemoved:
      s = removeTask(s, e.taskId);
      break;
  }
  return s;
}

// playground
var s: Store = {};
var e1: TaskAddedEvent = newTaskAdded({
  eventId: '12',
  taskId: '45',
  name: 'Clean dishes'
});
var e2: TaskRemovedEvent = newTaskRemoved({
  eventId: '34',
  taskId: '45'
});
console.log(JSON.stringify(s));
s = handleEvent(s, e1);
console.log(JSON.stringify(s));
s = handleEvent(s, e2);
console.log(JSON.stringify(s));

// lib
function objectAssign (target, source) {
  var from;
  var keys;
  var to = Object(target);

  for (var s = 1; s < arguments.length; s++) {
    from = arguments[s];
    keys = Object.keys(Object(from));

    for (var i = 0; i < keys.length; i++) {
      to[keys[i]] = from[keys[i]];
    }
  }

  return to;
};