一、闭包的概念:
闭包是一个拥有许多变量和绑定了这些变量的环境的表达式(通常是一个函数),因此这些变量也是该表达式的一部分。用通俗的话来说,JS中的所有function都是一个闭包,但嵌套的function产生的闭包更为强大,这也是大部分时候我们所谓的闭包。
二、闭包的微观世界:
要深入理解闭包以及函数a和嵌套函数b的关系,我们需要了解函数执行环境(excution content)、活动对象(call object)、作用域(scope)和作用域链(scope chain)的概念。以函数a从定义到执行的过程为例来阐述这几个概念:
当定义函数a的时候,JS解释器会将函数a的作用域链设置为“定义a时a所在的环境”,如果a是一个全局函数,则作用域链中只有window对象;
当执行函数a时,a会进入相应的执行环境;
在创建执行环境的过程中,首先会为函数啊添加一个scope属性,即a的作用域,其值就是第一步中的作用域链,即a.scope=a的作用域链;
然后执行环境会创建一个活动对象。活动对象也拥有一个属性对象,但它不具有原型而且不能通过javaScript直接访问。创建完活动对象后,把活动对象添加到a的作用域链的最顶端。此时a的作用域链包含两个对象:a的活动对象和window对象;
下一步是在活动对象上添加一个arguments属性,它保存着调用函数a时所传递的参数。
最后把所有函数a的形参和内部的函数b的引用也添加到a的活动对象上。在这一步中,完成了函数b的定义,因此如同第三步,函数b的作用域链被设置为b所被定义的环境,即a的作用域。
到此,整个函数a从定义到执行的步骤就完成了。此时a返回函数b的引用给c,又函数b的作用域链包含了对函数a的活动对象的引用,也就是说b可以访问到a中定义的所有变量和函数。函数b被c引用,函数b有依赖于函数a,因此函数a在返回后不会被GC收购。
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疑难解答