Question

我目前正在学习JS对象并尝试对属性执行基本减法，然后通过方法输出。但是，即使typeof告诉我currentAge和currentYear是数字，对象方法也会返回NaN的值。我做错了什么？我怎样才能得到以下方法来输出正确的数字？

＆＃13;

var today = new Date();
var year = today.getFullYear();

var Sam = {
  age: 27, //Number
  birthMonth: "March", //March
  currentYear: year, //number
  birthYear: this.currentYear - this.age,

  // Method to say birth year
  sayBirthYear: function(){
    console.log("I was born in the year " + this.birthYear );     
    //return NANS
  }
}

Sam.sayBirthYear(); // Outputs Birth year

＆＃13;

Answer 1

上下文game === 3不是引用当前对象switch，而是引用封闭上下文，在本例中是this上下文。

另一种方法是使用函数，并从中创建对象。这样，封闭的上下文就是函数：

Sam

Answer 2

this.currentYear - this.age执行时this不是Sam引用的对象。实际上，该对象尚未创建，currentYear和age在此时不能作为任何对象的属性。您可以通过几种方法来实现所需的行为。

您可以在创建对象后添加birthYear属性，如下所示：

＆＃13;

var today = new Date();
var year = today.getFullYear();

var Sam = {
  age: 27,
  birthMonth: "March",
  currentYear: year,

  // Method to say birth year
  sayBirthYear: function(){
    console.log("I was born in the year " + this.birthYear ); 
  }
}

Sam.birthYear = Sam.currentYear - Sam.age;

Sam.sayBirthYear(); // Outputs Birth year

＆＃13;

或者您可以在对象创建期间执行此操作：

＆＃13;

var today = new Date( );
var year = today.getFullYear( );

var Sam = new function ( ) {
  this.age = 27;
  this.birthMonth = "March";
  this.currentYear = year;
  this.birthYear = this.currentYear - this.age;

  // Method to say birth year
  this.sayBirthYear = function ( ) {
    console.log( "I was born in the year " + this.birthYear );
  };
};

Sam.sayBirthYear( ); // Outputs Birth year

＆＃13;

或者你可以生下这样的吸气剂：

＆＃13;

var today = new Date();
var year = today.getFullYear();

var Sam = {
  age: 27,
  birthMonth: "March",
  currentYear: year,
  get birthYear() { return this.currentYear - this.age; },

  // Method to say birth year
  sayBirthYear: function(){
    console.log("I was born in the year " + this.birthYear ); 
  }
}

Sam.sayBirthYear(); // Outputs Birth year

＆＃13;

Answer 3

在JavaScript中，this默认为window：

john = { whoami: this }
john.whoami // window

您必须将其放入一个能够更改其值的函数中：

function whoami () { return this; }
john = { whoami: whoami }
jack = { whoami: whoami }
whoami() // window
john.whoami() // john
jack.whoami() // jack
whoami.call(john) // john
whoami.call(jack) // jack

对对象项目执行基本数学但值返回NaN

3 个答案: