Implement difference(a, b) — elements in a not in b

Question

Accepted Answer

**Approach:** `Set` for O(n). For `differenceBy` support iteratee.

```javascript
function difference(a, b) {
  const setB = new Set(b);
  return a.filter(x => !setB.has(x));
}
function symmetricDifference(a, b) {
  const setA = new Set(a), setB = new Set(b);
  return [...a.filter(x=>!setB.has(x)), ...b.filter(x=>!setA.has(x))];
}
function differenceBy(a, b, iteratee) {
  const fn = typeof iteratee==='function'? iteratee : v=>v[iteratee];
  const setB = new Set(b.map(fn));
  return a.filter(x => !setB.has(fn(x)));
}

// Demo
console.log(difference([2,1,3], [2,3])); // [1]
console.log(symmetricDifference([1,2,3],[3,4,5])); // [1,2,4,5]
console.log(differenceBy([2.1,1.2],[2.3,3.4], Math.floor)); // [1.2]
console.log(difference([], [1,2])); // []
```
ES2024 `Set.prototype.difference` is native — mention.

Implement difference(a, b) — elements in a not in b

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