Omit matching object keys

Creates an object composed of the properties the given function returns falsy for.

• Use Object.keys() and Array.prototype.filter() to remove the keys for which fn returns a truthy value.
• Use Array.prototype.reduce() to convert the filtered keys back to an object with the corresponding key-value pairs.
• The callback function is invoked with two arguments: (value, key).

const omitBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => !fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});

omitBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { b: '2' }