test.js

const testObj = {
	b: {
		c: 1,
		d: 2,
		e: {
			aa: 'test',
		}
	},
	a: {
		be: 'abe',
		ce: 'ace'
	}
};


function objectEntriesToKeys(fromObject) {
	const results = {};

	const getKeyPath = (obj, thisKey = '') => {
		for (let key in obj) {
			if (thisKey) {
				thisKey += `_${key}`;
			} else {
				thisKey = key;
			}

			if (typeof obj[key] === 'object') {
				getKeyPath(obj[key], thisKey)
			} else {
				results[thisKey] = obj[key];
				thisKey = '';
			}
		}
	}

	getKeyPath(fromObject);

	return results;
};

objectEntriesToKeys(testObj);