Polyfill Array.prototype.map from scratch

Question

Accepted Answer

**Approach:** Loop by index, check `i in array` for holes, call callback with `thisArg`.

```javascript
Array.prototype.myMap = function(callback, thisArg) {
  if (this == null) throw new TypeError('myMap called on null');
  if (typeof callback !== 'function') throw new TypeError(callback + ' is not a function');
  const O = Object(this);
  const len = O.length >>> 0;
  const A = new Array(len);
  for (let i = 0; i < len; i++) if (i in O) A[i] = callback.call(thisArg, O[i], i, O);
  return A;
};

// Demo
console.log([1,2,3].myMap(x => x*2)); // [2,4,6]
console.log([1,,3].myMap(x => (x||0)+1)); // [2, empty, 4] preserves holes
console.log([1,2,3].myMap(function(x){ return x + this.n }, {n:10})); // [11,12,13]
console.log(Array.prototype.myMap.call({0:'a',1:'b', length:2}, x=>x.toUpperCase())); // ['A','B']
const sparse = [1,,3];
console.log(1 in sparse.myMap(x=>x)); // false (hole preserved)
```
Spec: uses `>>>0` to coerce length, skips holes, passes (value, index, array).

Polyfill Array.prototype.map from scratch

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