This关键字

This

what it 'this'

special variable that is created for every execution context.

this是在每一个执行上下文中产生的变量的，每一个执行上下文都有自己的this值。this不是静态的，它取决于函数如何被调用，它的值会在函数被调用的时候被赋予。

the value of 'this'

1. Method: this = < Object that is calling the method >
2. Simple function call: this = undefined
3. Arrow function: this = < this of surrounding function > (父函数的this指向)
4. Event Listener: this = < DOM element that the handle is attached to >
5. new, call, apply, bind: this = self