The key to understanding design patterns such as IFFEIIFE is to realize that until recently{{when|date=February 2019}} JavaScript only featured [[Scope (computer science)#Function scope|function scope]] (thus lacking [[Scope (computer science)#Block scope|block scope]]), passing [[Call_by_reference|values by reference]] inside [[Closure (computer science)|closure]]s.<ref>{{cite book |last=Haverbeke |first=Marijn |title=Eloquent JavaScript |year=2011 |publisher=No Starch Press |isbn=978-1-59327-282-1 |pages=29–30}}</ref> This is no longer the case, as the ES6 version of JavaScript implements block scoping using the new <code>let</code> and <code>const</code> keywords.<ref>ECMAScript 6: New Features: Overview & Comparison, [http://es6-features.org/#BlockScopedVariables Block-Scoped Variables]</ref>
