Scope and the problem with globals

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Questions
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1- What is scope? Your explanation should include the idea of global vs. local scope.
2- Why are global variables avoided?
3- Explain JavaScript's strict mode
4- What are side effects, and what is a pure function?

Answers
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1- JavaScript has two scopes – global and local. 
Any variable declared outside of a function belongs to the global scope, and is therefore accessible from anywhere in your code. 
Each function has its own scope, and any variable declared within that function is only accessible from that function 
and any nested functions. 
Because local scope in JavaScript is created by functions, it’s also called function scope. 
When we put a function inside another function, then we create nested scope.

2- The reason is that every JavaScript file included in the page runs in the same scope. 
If you have global variables or functions in your code, scripts included after yours that contain the same variable and function names 
will overwrite your variables/functions.
This practice will produce some undesirable results (side-effect)

3- Strict mode is a way of telling browsers (and JavaScript debuggers) to be, well, stricter about how they parse your code. 
Strict mode eliminates some JavaScript silent errors by changing them to throw errors.
The 'use strict' command can be put at the top of a file to enforce strict mode for the entire file, 
or at the top of a function, if you only want to use strict mode within the function body

4- 
 A "side effect" is any effect other than that return value
 A "pure function" is a function that maps its input value(s) into an output value