Searches the given object by the given predicate and returns the found key or undefined.

(
  predicate: (value: any, key: string, context: object) => boolean
) => (xs: object) => any

findKey(({ x }) => x % 2 === 0)({
  a: { x: 1 },
  b: { x: 2 },
  c: { x: 3 },
});
// ⇒ "b"

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