JavaScript - 理解方法

Question

我對理解方法原理有疑問。
我理解功能，我知道

function hello() {
    alert('Hello World');
}  

是相同的

var hello = function() {
    alert('Hello World');
}  

現在，我的問題是什么。
這是我的一個方法的對象。 我不明白為什么在yarsLeft內部function people()不需要括號
我正在尋找一個合乎邏輯的解釋。

function people(name, age) {
    this.name = name;
    this.age = age;
    this.yearsUntilRetire = yearsLeft; // this is confised for me
}

function yearsLeft() {
    var numYears = 65 - this.age;
    return numYears;
}

創建對象

var test = new people('Superman', 50);
test.yearsUntilRetire(); // I understand this code and calling a method in that way

為什么我不能寫this.yearsUntilRetire() = yearsLeft或this.yearsUntilRetire = yearsLeft();

Answer 1

當您使用沒有括號（ () ）的函數名稱時，您將設置對函數本身的引用。

當您使用與括號相同的功能時，您正在執行該功能 。

所以這一行

this.yearsUntilRetire = yearsLeft;

設置yearsUntilRetire指向函數。 此后你可以這樣做：

this.yearsUntilRetire(); // execute the yearsLeft function.

---

為什么我不能寫這個.yearsUntilRetire（）= yearsLeft或this.yearsUntilRetire = yearsLeft（）;

在第一種情況下，您不能將調用函數的結果設置為不同的結果 - 這是語法錯誤。

在第二種情況下，您可以 - 它將變量yearsUntilRetire設置為調用函數yearsLeft的結果

Answer 2

1） function hello() {alert('Hello World');} 與 var hello = function() {alert('Hello World');}

第一個是函數聲明。 第二個是分配給名為hello的變量的函數表達式。 但是當然在使用這兩個函數時，您可以將其視為相同且兩者的行為相同。

2）函數將始終被調用，如函數名稱后跟括號，例如： yearsLeft() 。 這是調用函數的唯一方法。

函數是JS中的First Class對象。 它可以聲明，表達，分配，傳遞給另一個函數並從函數返回。

所以在你的情況下，你的意圖是將全局函數yearsLeft object屬性yearsUntilRetire 。 你完成任何其他任務的方式 - 記住“ 函數是頭等對象 ”

Answer 3

這里有幾個工作示例來說明您的問題的答案。 請參閱內聯注釋以獲得解釋。

 console.assert( typeof helloWorldAsVar === 'function', 'helloWorldAsVar is defined' ); console.log( helloWorldAsVar ); console.assert( typeof helloWorld === 'function', 'helloWorld is defined' ); // creating a function like helloWorld means it will be hoisted to the top of the file as js does two passes // when compiling the script. one to declare all the functions and initialize all the variables as undefined. // and another to run through the code. console.log(helloWorld); var helloWorldAsVar = function() { return 'this will not get hoisted, and the function name is anonymous'; } // helloWorldAsVar is only available after it is declared. console.assert(typeof helloWorldAsVar === 'function', 'helloWorldAsVar is defined'); console.log( helloWorldAsVar ); function helloWorld() { return 'this will get hoisted to the top of the function, and it will keep its name'; } 

 <script src="http://codepen.io/synthet1c/pen/WrQapG.js"> </script> 

 // this is a constructor function, it is essentially a typed object that can // have it's own methods similar to an oop class in other languages like php or java // the convention is to name a constructor with the first letter capitalized and it should be singular function Person(name, age) { // here you are assigning properties to your People object this.name = name; this.age = age; // you are passing a reference to the yearsLeft function // this.yearsUntilRetire = yearsLeft; } // a more standard way to add the yearsLeft function is to // use the People.prototype object which get's methods delegated to it eg. Person.prototype.yearsUntilRetire = yearsLeft; function yearsLeft() { var numYears = 65 - this.age; return numYears; } // Create objects var superman = new Person('Clark', 50 ), batman = new Person('Bruice', 40 ); console.log( superman, 'yearsUntilRetire', superman.yearsUntilRetire() ); console.log( batman, 'yearsUntilRetire', batman.yearsUntilRetire() ); // Why I can 't write this.yearsUntilRetire() = yearsLeft or this.yearsUntilRetire = yearsLeft(); console.assert(this === window, 'this in the global context refers to the window object' ); console.assert(superman instanceof Person, 'superman is a Person' ); console.assert(batman instanceof Person, 'batman is a Person' ); 

 <script src="http://codepen.io/synthet1c/pen/WrQapG.js"></script>