deepFlattenJSObject

function flattenObject(objToFlatten) {
  let result = {};
  let objectsToFlatten = [objToFlatten];

  while (objectsToFlatten.length) {
    let currentObject = objectsToFlatten.pop();
    let currentKeys = Object.keys(currentObject);

    for (let i=0; i<currentKeys.length; i++) {
      let currentKey = currentKeys[i];
      let currentItem = currentObject[currentKey];
      if (typeof(currentItem) === 'object') {
        let childKeys = Object.keys(currentItem);
        let newChildObject = {};
        childKeys.forEach((key) => {
          newChildObject[`${currentKey}.${key}`] = currentItem[key];
        });
        objectsToFlatten.push(newChildObject);
      } else {
        result[currentKey] = currentItem;
      }
    }
  }

  return result;
}

let myObject = {
  a: 5,
  b: 6,
  c: {
    f: 9,
    g: {
      m: 17,
      n: 3
    }
  }
}

console.log(flattenObject(myObject)) // should produce {a: 5, b: 6, c.f: 9, c.g.m: 17, c.g.n: 3}