Is there a way to store a reference to a deep object key in a variable?

Question

Giving the following example:

const a = {
  b: {
    c: null
  }
};

let d = ["b", "c"];

let reference = a;

d.forEach(key => (reference = reference[key]));

reference = "fe";

console.log("With a reference to the object deep key.");
console.log(a);

a["b"]["c"] = "fe";
console.log("Directly setting it, what I want.");
console.log(a);

Would it be possible to change the value of c with a reference to it? This is the closest I am but it obviously stores the null value on it.

`let ref = a.b; ref.c = 'fe';` is the best you can do. `ref = ...` never modifies an existing object. — deceze, Mar 04 '20 at 13:01
Does this answer your question? [Is JavaScript a pass-by-reference or pass-by-value language?](https://stackoverflow.com/questions/518000/is-javascript-a-pass-by-reference-or-pass-by-value-language) — VLAZ, Mar 04 '20 at 13:03
Not directly. You can keep a reference one layer higher on `a['b']` for example, which is an Object and has a reference. — Andrew Radulescu, Mar 04 '20 at 13:18

adiga · Accepted Answer · 2020-03-04T13:30:02.260

1

You need to pop the last key from d and use it on reference at the end.

let d = ["b", "c"],
    last = d.pop(),
    reference = a;

path.forEach(key => reference = reference[key] || {});

reference[last] = "fe";

You can use reduce to get the object's reference in the accumulator and update the last key with the provided value

function updatePath(original, path, value) {
  const last = path.pop();
  path.reduce((acc, key) => acc[key] || {}, original)[last] = value;
  return original
}

const eg1 = {
  b: {
    c: null
  }
}

console.log(
  updatePath(eg1, ['b', 'c'], 'fe')
)

const eg2 = {
  1: {
    2: {
      3: 'initial'
    }
  }
}

console.log(
  updatePath(eg2, ['1', '2', '3'], 'updatedValue')
)

edited Mar 04 '20 at 13:30

answered Mar 04 '20 at 13:08

adiga

34,372
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The more generic concept of a configurable setter/getter is called *functional lenses* or just *lenses* (plural, or *lens* singular). [Here is an article on them](https://medium.com/javascript-scene/lenses-b85976cb0534). [Ramda has an implementation](https://ramdajs.com/docs/#lens). [Lodash also has one](https://lodash.com/docs/4.17.15#set) but doesn't call it a lens. – VLAZ Mar 04 '20 at 13:31

score 0 · Answer 2 · answered Mar 04 '20 at 13:00

0

You can only change values "by reference" for arrays and objects. But for objects and arrays, yes you can!

You can check this post that explains well the primitive values vs reference values.

answered Mar 04 '20 at 13:00

Ricardo Rocha

14,612
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Is there a way to store a reference to a deep object key in a variable?

2 Answers2