javascript - 调用 javascript 构造函数方法

Question

我在调用我创建的构造函数内部的方法时遇到问题。这是函数，我尝试调用它...

var Cohort = function(program, campus, number, students){
    this.program = program,
    this.campus = campus,
    this.number = number,
    this.students = students,
    function sayName(){
        return `This cohort is called ${program}, ${campus}, ${number}.`
    },
    function takeAttendance(){
        return console.log(students)
    }
}

var cohort1 = new Cohort("w", "pr", 27, ['devin', 'ben', 'bob'])
var cohort2 = new Cohort('w', 'pr', 31, ["Brendan Eich", "Dan Abramov", "Wes Bos", "Kent Dodds"])

cohort1.sayName()

控制台显示 cohort1.sayName 不是函数。

Answer 1

您必须将方法设置到原型(prototype)上。您在代码中所做的只是简单地将函数声明为 Cohort 函数范围的局部函数，因此它们不是方法。

每当您调用 new Cohort(...) 时，都会从 链接一个对象(new Cohort().__proto__ === Cohort.prototype) >Cohort.prototype，它成为新 Cohort 对象的 this，您的属性将保存到该对象中。设置 Cohort.prototype.methods 使用原型(prototype)链逻辑允许在 Cohort 对象的每个实例上调用这些方法

var Cohort = function(program, campus, number, students) {
    this.program = program
    this.campus = campus
    this.number = number
    this.students = students
}

Cohort.prototype.sayName = function() {
  return `This cohort is called ${this.program}, ${this.campus}, ${this.number}.`
}

Cohort.prototype.takeAttendance = function() {
  return console.log(this.students)
}

var cohort1 = new Cohort("w", "pr", 27, ['devin', 'ben', 'bob'])
var cohort2 = new Cohort('w', 'pr', 31, ["Brendan Eich", "Dan Abramov", "Wes Bos", "Kent Dodds"])

console.log(cohort1.sayName())
cohort1.takeAttendance()

console.log(Object.getOwnPropertyNames(cohort1))
console.log(Object.getOwnPropertyNames(cohort1.__proto__))

在 ES6 中，你可以简单地做

class Cohort {
  constructor(program, campus, number, students) {
    this.program = program
    this.campus = campus
    this.number = number
    this.students = students
  }

  sayName() {
    return `This cohort is called ${this.program}, ${this.campus}, ${this.number}.`
  }

  takeAttendance() {
    return console.log(this.students)
  }
}

var cohort1 = new Cohort("w", "pr", 27, ['devin', 'ben', 'bob'])
var cohort2 = new Cohort('w', 'pr', 31, ["Brendan Eich", "Dan Abramov", "Wes Bos", "Kent Dodds"])

console.log(cohort1.sayName())
cohort1.takeAttendance()

console.log(Object.getOwnPropertyNames(cohort1))
console.log(Object.getOwnPropertyNames(cohort1.__proto__))

这个 class 构造实际上只是一个语法糖，实现了与 ES5 中相同的逻辑

作为额外说明，以下代码也有效，但通常不是首选。注意方法存储位置的不同(检查最后 2 个 console.log 并与上面的进行比较)

var Cohort = function(program, campus, number, students) {
    this.program = program
    this.campus = campus
    this.number = number
    this.students = students
    
    this.sayName = function() {
  return `This cohort is called ${this.program}, ${this.campus}, ${this.number}.`
    }
    
    this.takeAttendance = function() {
      return console.log(this.students)
    }
}

var cohort1 = new Cohort("w", "pr", 27, ['devin', 'ben', 'bob'])
var cohort2 = new Cohort('w', 'pr', 31, ["Brendan Eich", "Dan Abramov", "Wes Bos", "Kent Dodds"])

console.log(cohort1.sayName())
cohort1.takeAttendance()

console.log(Object.getOwnPropertyNames(cohort1))
console.log(Object.getOwnPropertyNames(cohort1.__proto__))