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- There is NO class concept in JS, not like Java / C++, it is prototype based (i.e., object based).
- The primitive data in JS: string, number, boolean, and undefined; the complex data in JS: object, and function. So you may ask there is no array data structure in JS? JS does have array data structure, but it belongs to JS object (i.e., JS array is a type of JS object), there is no JS data type that is called array. (see here for more details.)
- JS Scope (JS Scope — W3Schools) – a very good post well explained the scope of JS function can be found at here (pdf)