实例化：A类依赖于B类。

class B {
  doSth () {

  }
}

class A {
  constructor () {}

  run () {
    const b = new B()
    b.doSth()
  }
}

new A().run()

委托：A对象依赖于B对象。

class B {
  doSth () {

  }
}
const b = new B()

class A {
  constructor () {}

  run () {
    b.doSth()
  }
}
new A().run()

聚合：A对象和B对象是"has-a"关系。

class Student {  
  constructor(name) {  
    this.name = name  
  }
}

class Classroom {  
  constructor() {  
    this.students = []  
  }

  addStudent(student) {  
    this.students.push(student)  
  }  
}

let student1 = new Student('Alice')  
let student2 = new Student('Bob')  
let classroom = new Classroom()  
classroom.addStudent(student1)  
classroom.addStudent(student2)

组合：A对象和B对象是"is-a"关系。

class Creature {  
  constructor(name) {  
    this.name = name  
  }  
}  

class Eye {  
  constructor(color) {  
    this.color = color  
  }  
}  

class Human extends Creature {  
  constructor() {  
    super('Human')  
    this.leftEye = new Eye('blue')  
    this.rightEye = new Eye('brown')  
  }  
}

继承：A类和B类是"is-a"关系。