1分钟部署网站📞AI智能客服,大模型训练自有数据,简单好用,有效降低客服成本 广告
在 JavaScript 中,如同在大部分面向对象编程语言中一样,`this` 是一个特殊的关键字,它常在被某个对象调用的方法中指向对象本身。`this` 的值可通过一系列简单的步骤来确定: * 如果函数是通过 `Function.call()` 或者 `Function.apply()` 调用,`this` 的值将会被设置为传递给 `.call()` 或 `.apply()` 的第一个参数。如果传递给 `.call()`或 `.apply()` 的第一个参数是 `null` 或 `undefined`,`this` 会指向全局对象(在 Web 浏览器中是 `window` 对象)。 * 如果被调用的函数是由 `Function.bind()` 创建的,`this` 将会是该函数被创建时传递给 `.bind()` 的第一个参数。 * 如果函数是作为一个对象的方法被调用,`this` 将会指向那个对象。 * 否则,当函数作为一个不依附任何对象的独立函数被调用,`this` 会指向全局对象。 ~~~ // A function invoked using Function.call() var myObject = { sayHello: function() { console.log( "Hi! My name is " + this.myName ); }, myName: "Rebecca" }; var secondObject = { myName: "Colin" }; myObject.sayHello(); // "Hi! My name is Rebecca" myObject.sayHello.call( secondObject ); // "Hi! My name is Colin" ~~~ ~~~ // A function created using Function.bind() var myName = "the global object"; var sayHello = function() { console.log( "Hi! My name is " + this.myName ); }; var myObject = { myName: "Rebecca" }; var myObjectHello = sayHello.bind( myObject ); sayHello(); // "Hi! My name is the global object" myObjectHello(); // "Hi! My name is Rebecca" ~~~ ~~~ // A function being attached to an object at runtime. var myName = "the global object"; var sayHello = function() { console.log( "Hi! My name is " + this.myName ); }; var myObject = { myName: "Rebecca" }; var secondObject = { myName: "Colin" }; myObject.sayHello = sayHello; secondObject.sayHello = sayHello; sayHello(); // "Hi! My name is the global object" myObject.sayHello(); // "Hi! My name is Rebecca" secondObject.sayHello(); // "Hi! My name is Colin" ~~~ 当深度调用一个长命名空间的函数时,它通常会诱使你使用一个单一简短的变量来引用实际的函数,以减少你所需要键入的代码。重点是实例方法不能这么做,因为这会导致函数内的 `this` 值发生改变,从而导致不正确的结果。例如: ~~~ var myNamespace = { myObject: { sayHello: function() { console.log( "Hi! My name is " + this.myName ); }, myName: "Rebecca" } }; var hello = myNamespace.myObject.sayHello; hello(); // "Hi! My name is undefined" ~~~ 但是,你可以安全的减少所有代码,直到方法被调用的那个对象: ~~~ var myNamespace = { myObject: { sayHello: function() { console.log( "Hi! My name is " + this.myName ); }, myName: "Rebecca" } }; var obj = myNamespace.myObject; obj.sayHello(); // "Hi! My name is Rebecca" ~~~