if 语句在函数中有效，但在 setter 方法中无效，反之亦然，setter 也根本不起作用

Question

我正在 CodeCademy 上做一个项目，我需要为一些学生创建一个 setter 方法。它应该检查该值是否是数字，如果是，则返回该值，否则记录错误。我最初想出了这段代码

set numberOFStudents(val) {
    if(typeof val === 'number') {
      this._numberOfStudents = val;
    } else {
      console.log('Invalid input: numberOfStudents must be set to a Number.') 
    }
  } 

，但它不起作用，因为该值没有效果。它将返回字符串和数字作为值。 但是，当我在常规函数中使用此方法时，它完美地工作 https://jsfiddle.net/蒙蒂耶克/v8zsL5et/12/ 本演练中使用的方法是这样的：

set numberOfStudents(val) {
    if(val.isNaN()) {
      console.log('Invalid input: numberOfStudents must be set to a Number.')
    } else {
       this._numberOfStudents = val;
    }
  }

这在常规函数中不起作用，因为它返回错误 https: //jsfiddle.net/Montinyek/e6k7qdcs/ 但更让我困惑的是，演练中使用的方法似乎也没有效果。我仍然可以将学生人数设置为字符串，它会记录它，而不是记录错误。这是整个代码的jsfiddle https://jsfiddle.net/Montinyek/1j7wL5gm/

Answer 1

这是因为您没有在构造函数中使用设置器。因此，当您传入字符串时，它会分配它。

您需要更改

  constructor(name, level, numberOfStudents) {
    this._name = name;
    this._level = level;
    this._numberOfStudents = numberOfStudents;
  }

To，

  constructor(name, level, numberOfStudents) {
    this._name = name;
    this._level = level;
    this.numberOfStudents = numberOfStudents;
  }

以便它使用 setter 而不是成员变量。第一种方式直接将其分配给您的成员，而不是使用 setter。

这是根据您的示例进行更改的完整代码；

class School {
  constructor(name, level, numberOfStudents) {
    this._name = name;
    this._level = level;
    this.numberOfStudents = numberOfStudents;
  }
  get name() {
    return this._name;
  }
  get level() {
    return this._level;
  }
  get numberOfStudents() {
    return this._numberOfStudents;
  }
  set numberOfStudents(val) {
    if(typeof val !== 'number') {
      console.log('Invalid input: numberOfStudents must be set to a Number.')
    } else {
       this._numberOfStudents = val;
    }
  }
  quickFacts() {
    console.log(`${this.name} educates ${this.numberOfStudents} students at the ${this.level} school level.`)
  }
  static pickSubstituteTeacher(substituteTeachers) {
    const randInd = Math.floor(Math.random() * substituteTeachers.length)
    return substituteTeachers[randInd];  
  }   
}

class PrimarySchool extends School {
  constructor(name, numberOfStudents, pickupPolicy) {
    super(name, 'primary', numberOfStudents)
    this._pickupPolicy = pickupPolicy;
  }
  get pickupPolicy() {
    return this._pickupPolicy;
  }
}

class HighSchool extends School {
  constructor(name, numberOfStudents, sportsTeams) {
    super(name, 'high', numberOfStudents)
    this._sportsTeams = sportsTeams;
  }
  get sportsTeams() {
    return this._sportsTeams;
  }
}

const lorraineHansburry = new PrimarySchool('Lorraine Hansbury', 'g', 'Students must be picked up by a parent, guardian, or a family member over the age of 13.');
lorraineHansburry.quickFacts();
const alSmith = new HighSchool('Al E. Smith', 415, ['Baseball', 'Basketball', 'Volleyball', 'Track and Field'])
alSmith.quickFacts();

这是保存的 jsfiddle，显示其工作

https://jsfiddle.net/6qoadu27/6/