这使用了一个reduce，里面有一个forEach来构建一个对象。你可以使用这个对象来通过id查找一个用户，或者你可以使用Object.values来获取一个所有用户的数组。

const projects = [{
    projectId: 'abc123',
    name: 'Project 1',
    userProjects: [
      {
        userId: 'xyz789',
        projectId: 'abc123',
        user: { userId: 'xyz789', email: 'user1@test.com' },
      },
      {
        userId: 'sdf012',
        projectId: 'abc123',
        user: { userId: 'sdf012', email: 'user2@test.com' },
      },
    ],
  },
  {
    projectId: 'def456',
    name: 'Project 2',
    userProjects: [
      {
        userId: 'xyz789',
        projectId: 'def456',
        user: { userId: 'xyz789', email: 'user1@test.com' },
      },
    ],
  },
];

const lookup = projects.reduce((acc, el) => {
  el.userProjects.forEach(up => {
    const id = up.userId;
    
    if (!acc[id]) {
      acc[id] = {
        userId: id,
        email: up.user.email,
        projects: []
      };
    }
      
    acc[id].projects.push({projectId: el.projectId, name: el.name});
  });
  
  return acc;
}, {});

console.log("single user -->", lookup["xyz789"]);

const output = Object.values(lookup);
console.log("all users -->", output);