Polyfill Array.prototype.filter from scratch

Question

Accepted Answer

**Approach:** Loop by index, check `i in O` for sparse, test callback, push if truthy.

```javascript
Array.prototype.myFilter = function(callback, thisArg) {
  if (this == null) throw new TypeError('myFilter 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 res = [];
  for (let i=0; i<len; i++) {
    if (i in O) {
      const val = O[i];
      if (callback.call(thisArg, val, i, O)) res.push(val);
    }
  }
  return res;
};

// Demo
console.log([1,2,3,4].myFilter(x => x%2===0)); // [2,4]
console.log([1,,3,4].myFilter(x => x>1)); // [3,4] (hole skipped)
console.log([1,2,3].myFilter(function(x){ return x > this.min }, {min:2})); // [3]
console.log(Array.prototype.myFilter.call({0:'a',1:'b',2:'c',length:2}, x=>x!=='a')); // ['b']
console.log([0,1,false,2].myFilter(Boolean)); // [1,2]
```
Spec: does not mutate, skips holes, passes (value, index, array) and honors this

Polyfill Array.prototype.filter from scratch

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Understand the problem

Attempt it yourself

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KEEP THESTREAK ALIVE.

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