(_.merge in ES6/ES7)Object.assign without overriding undefined values

Question

There is _.merge functionality in lodash. I want to achieve the same thing in ES6 or ES7.

Having this snippet:

Object.assign({}, {key: 2}, {key: undefined})

I want to receive {key: 2}. Currently I receive {key: undefined}

This is NOT a deep merge.

Is it possible? If yes then how to achieve that?

Answer 1

You can't achieve that with a straight usage of Object.assign, because each next object will rewrite the same keys for prev merge. The only way, to filter your incoming objects with some hand-crafted function.

function filterObject(obj) {
    const ret = {};
    Object.keys(obj)
        .filter((key) => obj[key] !== undefined)
        .forEach((key) => ret[key] = obj[key]);
    return ret;
}

Answer 2

You can simply filter out the keys with undefined values before passing them to Object.assign():

const assign = (target, ...sources) =>
  Object.assign(target, ...sources.map(x =>
    Object.entries(x)
      .filter(([key, value]) => value !== undefined)
      .reduce((obj, [key, value]) => (obj[key] = value, obj), {})
  ))

console.log(assign({}, {key: 2}, {key: undefined}))

Answer 3

Write a little utility to remove undefined values:

function removeUndefined(obj) {
  for (let k in obj) if (obj[k] === undefined) delete obj[k];
  return obj;
}

Then

Object.assign({}, {key: 2}, removeUndefined({key: undefined}))

This seems preferable to writing your own assign with wired-in behavior to remove undefined values.

Answer 4

use lodash to omit nil values and then combine the two objects into one via spread

{ ...(omitBy({key: 2}, isNil)), ...(omitBy({key: undefined}, isNil))}

See more info on lodash here https://lodash.com/docs/4.17.15

Answer 5

With ES2019/ES10's new object method, Object.fromEntries(), Michał's answer can be updated:

const assign = (target, ...sources) =>
  Object.assign(target, ...sources.map(x =>
    Object.fromEntries(
      Object.entries(x)
        .filter(([key, value]) => value !== undefined)
    )
  ))

console.log(assign({}, {key: 2}, {key: undefined}))

Answer 6

If you just need the values and don't need an object, you could also use object destructuring:

const input = { a: 0, b: "", c: false, d: null, e: undefined };
const { a = 1, b = 2, c = 3, d = 4, e = 5, f = 6 } = input;
console.log(a, b, c, d, e, f);
// => 0, "", false, null, 5, 6

This will only override absent or undefined values.

I often use this for function argument default values like this:

function f(options = {}) {
  const { foo = 42, bar } = options;
  console.log(foo, bar);
}

f();
// => 42, undefined

f({})
// => 42, undefined

f({ foo: 123 })
// => 123, undefined

f({ bar: 567 })
// => 42, 567

f({ foo: 123, bar: 567 })
// => 123, 567