In-depth understanding of left query and right query in javascript

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Value and reference

Scope

• In a narrow sense, a scope is an object (more specifically, it should be a collection);

• In a broad sense, a scope is It is a set of rules for storing variables and making it easy to find these variables later;
  

• The scope is responsible for collecting and maintaining the scope consisting of all declared identifiers A series of queries and enforces a very strict set of rules that determines the access rights of the currently executing code to these identifiers.

Context

• Preparation before execution of code (global code, function body, eval code):
  

• 1. Promotion (variable function function expression);

• 2. Determine the point of this;

• 3. Associated with the corresponding scope;

• If the code segment is a function body, then on this basis, additional: parameter assignment, arguments assignment;

• The relationship between scope and execution context:
  

• A scope may contain several contexts. It is possible that there has never been a context (the function has never been called); it is possible that there has been, and now after the function is called, the context is destroyed;

Left/right query

• Left query:

• On the left side of the assignment symbol, who is the target of the assignment operation;

• The relationship between the actual and formal parameters when the function is called is a left query;

• The query of the variable on the left side of the equal sign. In the entire scope chain, if no declaration of a variable is found, the js engine will automatically declare a variable with the same name globally; however, the declaration of this variable will not be promoted.
  

  (function(){
      function test(a){
          b=a;
          console.log(b);//2
      }
      test(2);
  })();
  console.log(b);//2

• Right query:

• The non-left side of the assignment symbol, who is the source of the assignment operation ;

• Query for variables other than the left side of the equal sign. In the entire scope chain, if no declaration of a variable is found, a ReferenceError error will be thrown directly;

  console.log(a);//ReferenceError: a is not defined

• Special right query

  //a并未定义赋值
  console.log(typeof a);//undefined

• Strict mode & non-strict mode

• ##RHS and LHS will throw ReferenceError exception

• ReferenceError exception is the same as scope determination failure Related, TypeError means that the scope judgment is successful, but the operation on the result is illegal or unreasonable;

• The right query cannot be found in all nested scopes of the query If the required variables are required, the engine will throw a ReferenceError exception, but it should be noted that no exception will be reported when typeof is used;

• The left query cannot be found in all nested scopes of the query For the required variable, a variable with that name will be created in the global scope;

• If the variable has not been declared (the variable cannot be found in any scope) Below, the behavior of these two queries is different

• Non-strict mode:

  
  

• Strict mode:

  
  

• typeof's security prevention mechanism

• 1. Use the global variable DEBUG in the program as the "debug mode" switch, Our declaration in the DEBUG.js file var DEBUG =true; this file is only loaded into the browser during development and testing, and will not be loaded in the production environment;

  if（DEBUG）{ //在生产环境中会报错
           console.log("开始调试");
    }  if（typeof DEBUG !== "undefined"）{
            console.log("开始调试");    
    }

• 2 .Write a polyfill (padding code to supplement the missing functions in the current running environment) for a missing function

  if(typeof polyfill_a === "undefined"){        //注意这一块不需要var，跟变量的提升有关
             //这一块需要使用函数表达式而不是函数声明
             polyfill_a = function(){
                //功能代码
             }
     }

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