nested-object-flatten-keys.js

const theObject = {
    nested1: {
        key1: "value1",
        nested2: {
            key2: "value2"
        }
    },
    key3: "value3",
    key4: ["test", "test1"]
};

function flatKeys(obj, prefix = "") {
    return Object.keys(obj).reduce(function (res, el) {
        // uncomment this block if don't want array indexes
        // if (Array.isArray(obj[el])) {
        //     return [...res, el];
        // }
        if (typeof obj[el] === "object" && obj[el] !== null) {
            return [...res, ...flatKeys(obj[el], prefix + el + ".")];
        }
        return [...res, prefix + el];
    }, []);
}

const objectKeys = flatKeys(theObject)

console.log(JSON.stringify(objectKeys, null, 2))
/**
 * [
 *  "nested1.key1",
 *  "nested1.nested2.key2",
 *  "key3",
 *  "key4.0",
 *  "key4.1"
 * ]
 */