Symbol.hasinstance.js

## Symbol.hasinstance.js
/* Well Known Symbols : Symbol.hasinstance

sources:
1 - https://www.keithcirkel.co.uk/metaprogramming-in-es6-symbols/
2 - http://exploringjs.com/es6/ch_symbols.html
3 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof
4 - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/hasInstance

Well Known Symbols are Reflection via implementation in ES6.
Reflection in Metaprogramming: observing low level internals of a programming language/program.

One example of Well Known Symbols in JS is Symbol.hasInstance, which instanceof uses under the hood.
*/

// using instanceof :

function ClassyFunc() {}

let c = new ClassyFunc();

if (c instanceof ClassyFunc) {
  console.log('instanceof.')
}
else {
  console.log('not instanceof.')
}

// > instanceof

/* Let's implement that by hand.

Under the hood, instanceof uses (rhs)[Symbol.hasInstance](lhs),
where rhs is right hand side and lhs is left hand side.

Symbol is static scoped.
*/

class ArrayClass {
  static [Symbol.hasInstance](input) {
    return Array.isArray(input)
  }
}

let a = []

let b = 3

console.log(a instanceof ArrayClass)
// > true

console.log(a instanceof ArrayClass)
// > false