JS higher order --- the scope and the scope chain

Outline:

 

 

main body:

(1) Introduction

　　

 

(2) Level

+ Level execution context is n- . 1 Principle 
scope hierarchy is n- + 1'd principles

 

verification:

 

 

 (3) Function scope effect

Isolate variables in different scopes, the same variable names do not conflict

 

（4）

 

 

 

 

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