creates real genuine Arrays that are immutable but "polyfill" mutating methods

ImmutableArray.js

// ccby@dandavis

function ImmutableArray(){

  function odp(k,v){ Object.defineProperty(r,k,{value:v}); }

  let r = [...arguments];
  if(r.length===1 && Array.isArray(r[0])) r = r[0];

  odp("push", function(){return ImmutableArray(r.concat(...arguments));});
  odp("unshift", function(){return ImmutableArray(r.toSpliced(0,0,...arguments));});
  odp("splice", x=>ImmutableArray(r.toSpliced()));
  odp("sort", x=>ImmutableArray(r.toSorted()));
  odp("reverse", x=>ImmutableArray(r.toReversed()));
  odp("fill", function(){return ImmutableArray(r.slice().fill(...arguments));});
  odp("copyWithin", function(){return ImmutableArray(r.slice().copyWithin(...arguments));});
  odp("pop", x=>r.slice(-1)[0]);
  odp("shift", x=>r[0]);
  odp("length", r.length);
  Object.freeze(r);
  return r;
}// end ImmutableArray()

let r = new ImmutableArray([11,22,33]);
r.push(55).fill(66,1,3); // == [11,66,66,55]
r; // == [11,22,33]