<aside> ⚠️ This note serves as a reminder of the book's content, including additional research on the mentioned topics. It is not a substitute for the book. Most images are sourced from the book or referenced.

</aside>

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Infor & Preface

Chap 6 — Limiting Scope Exposure

Attention: decisions and patterns we apply across the whole program.

Least Exposure

• How and why we use different levels of scopes (functions and blocks)? ← functions create their own scope, it’s normal but why we need blocks?
• In Software Engineering, we have The Principle of Least Privilege (POLP - a concept in computer security that limits users' access rights to only what is strictly required to do their jobs) → its variation in this section is “Least Exposure” (POLE).
• Following POLE, we want to minimize the exposure of variables in each scope.
• [Bad idea] Placing all variables in the global scope, why?
  1. Naming Collisions: Identical names for variables or functions can lead to collisions, resulting in bugs.
  2. Unexpected Behavior: If access is granted to parts that are supposed to be private, other individuals may perform actions you did not intend.
  3. Unintended Dependency: If developers outside of your program create a dependency on some "supposed to be private" parts of your program, future changes to these parts could unexpectedly affect these external dependencies.

→ Exposing min necessary, keeping everying else as private as possible.

• In order to block the variable to the block scope, use let or const, don’t use var!

  function diff(x, y) {
    console.log(tmp); // ReferenceError with "const" and "let", undefined with "var"
    if (x > y) {
      const tmp = x; // or let or var
      x = y;
      y = tmp;
    }
    return y - x;
  }
  

Hiding in Plain (Function) Scope

• Use let or const to block the scope to lowest level but how to hide var or function too?

• Hide var by wrapping it by a function.

  var cache = {} // will be in the global scope
  function factorial(x) {
  	if (x < 2) return 1; if (!(x in cache)) {
  		cache[x] = x * factorial(x - 1);
  	}
  	return cache[x];
  }
  factorial(6); // 720
  

  // outer scope
  function hideTheCache() {
  	// middle scope
  	var cache = {}
  	function factorial(x) {
  		if (x < 2) return 1; if (!(x in cache)) {
  			cache[x] = x * factorial(x - 1);
  		}
  		return cache[x];
  	}
  }
  var factorial = hideTheCache();
  factorial(6); // 720
  

  var factorial = (function hideTheCache() {
  	var cache = {}
  	function factorial(x) {
  		if (x < 2) return 1; if (!(x in cache)) {
  			cache[x] = x * factorial(x - 1);
  		}
  		return cache[x];
  	}
  })() // () means "immediate invoke the function"
  

❇️ Invoking Function Expressions Immediately

The last () in the previous code is call Immediately Invoked Function Expression (IIFE). It’s useful to create a scropt to hide var/func.

// standalone IIFE -> "()" surround function is required
(function(){ .. })();

// with a name -> "()" surround function isn't required
(function namedIIFE(){ .. })();
function nameIIFE(){...}()