How to delete recursively undefined properties from an object - while keeping the constructor chain?

Question

This is a question similar to How to remove undefined and null values from an object using lodash?. However, the solutions proposed there do not conserve the constructor. In add

Answer 1

Disclaimer: I have no idea what lodash supports as built-in functions, but this is very easy to implement with vanilla javascript.

Start with a generic function for filtering your object's keys

// filter obj using function f
// this works just like Array.prototype.filter, except on objects
// f receives (value, key, obj) for each object key
// if f returns true, the key:value appears in the result
// if f returns false, the key:value is skipped
const filterObject = f=> obj=>
  Object.keys(obj).reduce((res,k)=>
    f(obj[k], k, obj) ? Object.assign(res, {[k]: obj[k]}) : res
  , {});

Then a function that filters based on your specific behavior

// filter out keys starting with `_` that have null or undefined values    
const filterBadKeys = filterObject((v,k)=> /^[^_]/.test(k) || v !== null && v !== undefined);

Then call it on an object

filterBadKeys({a: null, _b: null, _c: undefined, z: 1});
//=> { a: null, z: 1 }

This can be easily integrated into your constructor now

function Cons(obj) {
  _.extend(this, filterBadKeys(obj));
  // ...
}

---

EDIT:

On second thought, instead of butchering a perfectly good function with implicit deep recursion, you could abstract out the generic operations and define a specific "deep" filtering function

const reduceObject = f=> init=> obj=>
  Object.keys(obj).reduce((res,k)=> f(res, obj[k], k, obj), init);

// shallow filter      
const filterObject = f=>
  reduceObject ((res, v, k, obj)=> f(v, k, obj) ? Object.assign(res, {[k]: v}) : res) ({});

// deep filter     
const deepFilterObject = f=>
  reduceObject ((res, v, k, obj)=> {
    if (f(v, k, obj))
        if (v && v.constructor === Object)
            return Object.assign(res, {[k]: deepFilterObject (f) (v)});
        else
            return Object.assign(res, {[k]: v});
    else
        return res;
  }) ({});

const filterBadKeys = deepFilterObject((v,k)=> /^[^_]/.test(k) || v !== null && v !== undefined);

filterBadKeys({a: null, _b: null, _c: undefined, _d: { e: 1, _f: null }, z: 2});
//=> { a: null, _d: { e: 1 }, z: 2 }

Integration into your constructor stays the same

function Cons(obj) {
  _.extend(this, filterBadKeys(obj));
  // ...
}