30 seconds of code

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Returns the last key that satisfies the provided testing function. Otherwise undefined is returned.

Use Object.keys(obj) to get all the properties of the object, Array.prototype.reverse() to reverse their order and Array.prototype.find() to test the provided function for each key-value pair. The callback receives three arguments - the value, the key and the object.

const findLastKey = (obj, fn) =>
    .find(key => fn(obj[key], key, obj));