index.js

const { GraphQLServer } = require('graphql-yoga');

let count = 2;
let todos = [{
  id: '0',
  content: 'Buy milk',
  isCompleted: true
},
{
  id: '1',
  content: 'Cook some lobster',
  isCompleted: false
}];

const resolvers = {
  Query: {
    allTodos: () => {
      return todos;
    },
    Todo: (_, { id }) => {
      const todo = todos.find(x => x.id === id);
      if (!todo) {
        throw new Error('Cannot find your todo!');
      }
      return todo;
    }
  },
  Mutation: {
    createTodo: (_, { content, isCompleted }) => {
      const newTodo = {
        id: count++,
        content,
        isCompleted
      }
      todos = [...todos, newTodo];
      return newTodo;
    },
    updateTodo: (_, { id, content, isCompleted }) => {
      let updatedTodo;

      todos = todos.map(todo => {
        if (todo.id === id) {
          updatedTodo = {
            id: todo.id,
            // for content and isCompleted, we first check if values are provided
            content: content !== undefined ? content : todo.content,
            isCompleted: isCompleted !== undefined ? isCompleted : todo.isCompleted
          }
          return updatedTodo;
        } else {
          return todo
        }
      });

      return updatedTodo;
    },
    deleteTodo: (_, { id }) => {
      const todoToDelete = todos.find(x => x.id === id);

      todos = todos.filter(todo => {
        return todo.id !== todoToDelete.id;
      });

      return todoToDelete;
    }
  }
}

const options = {
  port: 3333,
  endpoint: '/graphql',
  subscriptions: '/subscriptions',
  playground: '/playground',
}

const server = new GraphQLServer({
  typeDefs: './src/schema.graphql',
  resolvers
})

server.start(options, ({ port }) =>
  console.log(
    `Server started, listening on port ${port} for incoming requests.`,
  ),
)