Notes from Scope & Closures by Kyle Simpson(O'Reilly) | © 2014 Kyle Simpson 978-1-449-33558-8
In Progress
In Traditional compiled-language process, code/program will undergo the following steps before it executed.
1. Tokenizing/Lexing
Breaking up a string of characters into meaningful chunks
2. Parsing
Taking a stream of tokens & turning into a tree of nested elements; which collectively represent the grammatical structure of the program. This tree is called as "AST - Abstract Syntax Tree"
3. Code-Generation
Process of taking an AST & turning it into executable code.
var a = 2;Engine
Compiler
Scope
The Cast
Responsible for start-to-finish compilation & execution of program
One of engine's friends; handles all the dirty work of parsing & code-generation
Another friend of Engine; Collects & maintains a look-up list of all the declared identifiers(var); enforces strict set of rules as to how these are accessible
Lets breakdown how Engine & Friends will approach the program,
var a = 2;A reasonable assumption could be summed by this pseudo code:
1. ALLOCATE MEMORY FOR A VARIABLE, LABEL IT a 2. ASSIGN THE VALUE 2 INTO THE VARIABLE
Unfortunately that's not quite accurate. Compiler will instead process as:
1. Encountering var a, Compiler asks Scope to see if a variable a already exists for that particular scope collection. If so, compiler ignores this declaration and moves on. Otherwise Compiler asks Scope to declare a new variable a for that scope.
2. Compiler then produces code for Engine to later execute, to handle the a = 2 assignment. The code Engine runs will first ask Scope if there is a variable called a accessible in the current scope collection. If so, Engine uses that variable. If not, Engine looks elsewhere (Nested Scope).
var a = 2;To Summarize: 2 Distinct actions are taken for a Variable Assignment
1. Compiler declares a variable(if not previously declared) in the current scope
2. When executing, Engine looks up the variable in Scope & assigns to it, if found