Question

我运行时有这段代码显示未定义。但是，我们可以使用此关键字访问全局属性。

 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

如果正确执行，则此方法有效（将alert替换为console.log以获得更简单的示例）

＆＃13;

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();

＆＃13;

如果将置于功能范围，它将无法正常工作，但是 - 大概是JS Fiddle做了类似的事情

＆＃13;

(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();
})();

＆＃13;

请注意，如果您有strict mode enabled，则this将undefined而不是window，代码将产生错误

＆＃13;

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();

＆＃13;

Answer 2

Normaly使用class ResourceDTO { // ... public void setResourceName(final String name) { this.resourceName = name; this.associateResource.forEach(a -> a.setAssociateName(name)); } }关键字。 window独立于声明函数的位置，但取决于调用的位置（以及如何调用）。

this

Answer 3

所以我开始知道，当我们使用严格模式时，此关键字在全局函数中保存undefined的值。但是，在严格模式下，它的值保持在进入执行上下文时设置的值，因此，在下面的例子中，这将默认为undefined：

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

    f2() === undefined;

因此，在严格模式下，如果执行上下文没有定义它，它仍然是未定义的。我从MDN获取了这段代码片段。

所以在我的情况下，价值并没有出现在小提琴中。但这将是@vlaz指出的理由。谢谢@vlaz

为什么我得到不明确的价值

3 个答案: