爲什麼我得到undefined值

Question

我有這段代碼，當我運行它顯示未定義。但是，我們可以使用此關鍵字訪問全球資產。

var firstName = "Peter", 
     lastName = "Ally"; 

     function showFullName() { 
     // "this" inside this function will have the value of the window object 
     // because the showFullName() function is defined in the global scope, just like the firstName and lastName 
     alert (this.firstName + " " + this.lastName); 
     } 
     showFullName(); 

Answer 1

通常使用window關鍵字代替。 this是獨立的地方是功能宣佈，但依賴於（以及如何）被稱爲的地方。

var firstName = "Peter", 
    lastName = "Ally"; 

function showFullName() { 
    alert (window.firstName + " " + window.lastName); 
} 
showFullName(); 

Answer 2

所以我才知道，當我們使用嚴格模式時，這個關鍵字在全局函數中保存了未定義的值。 在嚴格模式，但是，這個值保持在任何它被設置在進入執行上下文的時候，所以，在以下情況下，這將默認爲未定義：

function f2(){ 
     "use strict"; // see strict mode 
     return this; 
    } 

    f2() === undefined; 

所以，在嚴格模式下，如果這不是由執行上下文定義的，它仍然是未定義的。我已經從MDN獲取了這個代碼片段。

因此，在我的情況下，值不會出現在小提琴中。但它會是由@vlaz指出的原因。由於@vlaz

Answer 3

這工作如果正確執行（更換alert與console.log更容易的例子）

var firstName = "Peter", 
 
    lastName = "Ally"; 
 

 
function showFullName() { 
 
    // "this" inside this function will have the value of the window object 
 
    console.log("this and window are the same thing", this === window); 
 
    
 
    // because the showFullName() function is defined in the global scope, just like the firstName and lastName 
 
    console.log(this.firstName + " " + this.lastName); 
 
} 
 

 
showFullName();

如果這是放置在功能範圍它不會工作，但是 - 大概JS小提琴做那樣的事

(function() { 
 
    var firstName = "Peter", 
 
     lastName = "Ally"; 
 

 
    function showFullName() { 
 
     // "this" inside this function will still have the value of the window object 
 
     console.log("this and window are the same thing", this === window); 
 
     
 
     // however firstName and lastName are not goint to be attached to it because they are in functional scope 
 
     console.log("the names are still reachable", firstName, lastName) 
 
     
 
     //but not attached to the window object (which "this" points to) 
 
     console.log(this.firstName + " " + this.lastName); 
 
    } 
 

 
    showFullName(); 
 
})();

請注意，使用，如果你有strict mode enabled然後this將undefined，而不是window和代碼會產生一個錯誤

var firstName = "Peter", 
 
    lastName = "Ally"; 
 

 
function showFullName() { 
 
    "use strict"; 
 
    // "this" inside this function will now have the value "undefined" 
 
    console.log("'this' is actually 'undefined'", this); 
 
    console.log("the actual value 'undefined', not a string", typeof this); 
 
    
 
    // the following line will throw a TypeError because it's trying to get a property from "undefined" 
 
    console.log(this.firstName + " " + this.lastName); 
 
} 
 

 
showFullName();