Scope&Closure.md

Compiler Theory

Step of compiling:

1. Tokenizing/Lexing: breaking up a string of characters into tokens.
2. Parsing: taking a stream (array) of tokens and turning it into a tree of nested elements (Abstract Syntax Tree).
3. Code-Generation: the process of taking an AST and turning it into executable code.

Scope:

1. Engine: responsible for start-to-finish compilation and execution of our JavaScript program.
2. Compiler: handles all the work of parsing and code-generation.
3. Scope: collects and maintains a look-up list of all the declared identifiers (variables), and enforces a strict set of rules as to how these are accessible to currently executing code.

How to cheat Lexical scope? There are two ways: eval(..) and with, but it's a very bad idea, because of it defeats the Engine's ability to perform compile-time optimizations

Units of scope:

1. Functions
2. let/const from ES6 with blocks ({})
3. try/catch structure

Hoisting:

• All declarations are moved to their respective scopes

! Function declarations are hoisted before normal variables

Closure:

Closure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope.

Closure with loops (setTimeout task):

Wrong code:

for (var i=1; i<=5; i++) {
	setTimeout( function timer(){
		console.log( i );
	}, i*1000 );
}

Right code:

for (var i=1; i<=5; i++) {
	(function(j){
		setTimeout( function timer(){
			console.log( j );
		}, j*1000 );
	})( i );
}

Or

for (let i=1; i<=5; i++) {
	setTimeout( function timer(){
		console.log( i );
	}, i*1000 );
}