this points to the problem

The pointing of this cannot be determined when the function is defined. Only when the function is executed can it be determined who this points to. In general, the final pointing of this is the object that called it.

At this stage, let's take a look at a few this pointers

1. In the global scope or in ordinary functions, this points to the global object window (note that this in the timer points to window)

In the function of the first parameter of the timer method, this also points to the window object

2. In the method, whoever calls the method, this points to whom

3. This in the constructor points to the newly created instance