为什么我得到undefined值

Question

我有这段代码，当我运行它显示未定义。但是，我们可以使用此关键字访问全球资产。

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关键字代替。 this是独立的地方是功能宣布，但依赖于（以及如何）被称为的地方。

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

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

Answer 2

所以我才知道，当我们使用严格模式时，这个关键字在全局函数中保存了未定义的值。 在严格模式，但是，这个值保持在任何它被设置在进入执行上下文的时候，所以，在以下情况下，这将默认为未定义：

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

    f2() === undefined; 

所以，在严格模式下，如果这不是由执行上下文定义的，它仍然是未定义的。我已经从MDN获取了这个代码片段。

因此，在我的情况下，值不会出现在小提琴中。但它会是由@vlaz指出的原因。由于@vlaz

Answer 3

这工作如果正确执行（更换alert与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();

如果这是放置在功能范围它不会工作，但是 - 大概JS小提琴做那样的事

(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(); 
 
})();

请注意，使用，如果你有strict mode enabled然后this将undefined，而不是window和代码会产生一个错误

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();