JS variable defines the difference between Var and no Var in--fn

A global object is built into the JS runtime.

This global object is related to the operating environment. In the browser runtime environment. Global is the Window object.
In the Nodejs. The global object is a global object.

When you use a non-defined variable directly in the browser environment,
For example foo=123; foo, the variable is automatically declared as a global variable. The variable reference is automatically attached to the global object, which is the Window object, and you can try to verify it by using the properties that are equivalent to the globals.

Such as:

<! DOCTYPE html>
　　
In the body of the function defined under the Global object, without the variable declared by Var, first, it tries to chain in the current scope (as declared in the method, then the current scope chain represents the global scope and the method local scope etc ... ) to parse the variable; If the variable is found in any current scope chain, the variable is executed, and if the variable is not found, it is defined as a global variable and assigned to a global object (that is, the topmost object of the current scope chain, such as a Window object).
The test code is as follows:
<! DOCTYPE html>
So it is recommended that in FN Remember to define local variables to VAR, global variables do not VAR (note that there is no variable with the same name in the scope chain).
JS variable defines the difference between Var and no Var in--fn