Some problems encountered in detecting the existence of variables in JavaScript_javascript skills

When writing JavaScript programs, you often need to detect whether a certain variable exists. This is a very simple task, but you will encounter some problems if you are not careful. There are some main points:

1. Ordinary variables

Copy code The code is as follows :

2. Object properties

Copy code The code is as follows:

There is no need to use typeof to detect the properties of the object.

3. When adding attributes to objects, you will also encounter similar problems

Copy code The code is as follows: