Using Symbols to extend native prototypes in a way that cannot possibly clash with native extensions added in the future

symbol.based.extensions.js

const e = {};//define a store of possible props we want to add

(function(e){
  const empty = Symbol('empty');
  const _empty = ()=>[];
  
  const flatten = Symbol('flatten');
  function _flatten(){
    return this.reduce( (a,b) => a.concat(b), []);
  }
  
  Object.defineProperty(Array.prototype, empty,{
    value: _empty,
    writable: true,
    configurable: true,
    enumerable: false
  });

  Object.defineProperty(Array.prototype, flatten,{
    value: _flatten,
    writable: true,
    configurable: true,
    enumerable: false
  });
  
  Object.assign(e,{empty,flatten});
})(e);

//test data
const testArr = [4,5,6];
const testArr2 = [4,5,6,[5,6,7],[3,4]];

//usage: we're calling method that are defined on a Symbol key, using the prop lookup
testArr[e.empty]();//-> []
testArr2[e.flatten]();//-> [4, 5, 6, 5, 6, 7, 3, 4]