Review of JavaScript knowledge points (XXIII) -- LHS reference and RHS reference

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Compiler principle

JavaScript Usually classified as “ dynamic ” or “ Explain to perform ” Language , But it's actually a compiled language . But it is different from the traditional compilation language , It's not precompiled , The compiled results can not be transplanted in the distributed system . For all that ,JavaScript The compilation steps of the engine are very similar to those of traditional compilation languages . In some aspects, it may be more complicated than expected . In the process of traditional compiled language , A section of source code in a program goes through three steps before execution , Collectively referred to as “ compile ”：

1. participle / Lexical analysis ： Decompose a string of characters into （ For programming languages ） Meaningful code blocks , These blocks of code are called lexical units .
2. analysis / Syntax analysis ： Flow lexical units into （ Array ） Convert to a tree that represents the syntax structure of a program, which is composed of elements that are nested level by level （ Abstract syntax tree :AST）.
3. Code generation : take AST Convert to executable code .

JS Compiler principle

• engine ： To be responsible for the whole JavaScript Program compilation and execution process .
• compiler ： Responsible for syntax analysis and code generation .
• Scope ： Responsible for collecting and maintaining all variables ( identifier ) A series of queries , And implement a set of rules , Determine the current code execution access to these identifiers .

for example ： The assignment of a variable performs two actions ：

1. The compiler declares a variable in the current scope （ If it has not been stated before ）
2. At run time , The engine looks for the variable in the scope , If it can be found, it will be assigned .

that , How does the engine find variables ？

LHS Query and RHS Inquire about

LHS（left-hand Side） Query and RHS（right-hand Side） Inquire about , It usually refers to the reference of the left and right sides of the assignment operation .

When the variable appears on the left side of the assignment operation LHS Inquire about （ Find the storage location and assign a value ）, On the right RHS Inquire about （ Find its specific value ）

LHS and RHS The meaning is “ Left or right side of assignment operation ” It doesn't necessarily mean “= The left or right side of the assignment operator ”.

There are several other forms of assignment , So conceptually it's best to understand it as “ Who is the target of the assignment operation （LHS）” as well as “ Who is the source of assignment operation （RHS）”

for instance ：

    function foo(a) {
	var b = a;
	return a + b;
    }

    var c = foo(2);

 Copy code 

LHS Inquire about ：

1. c=... ,c Is the target of the assignment operation , So for c need LHS Inquire about .
2. Hidden a=2( Implicit variable assignment ), Calling foo(2) When , You need to take the arguments 2 Assign a value to a formal parameter , So for a need LHS Inquire about
3. b=.... , Same as 1

*RHS Inquire about

1. foo(2) ,foo(2) On the right side of the assignment operation , Need to know foo(2) Value , So it is RHS Inquire about
2. =a,a On the right side of the assignment operation , Need to know a Value , Yes a Conduct RHS Inquire about
3. a..., stay return a+b in , Need to know a and b Value , Should be right a and b separately RHS Inquire about .