Destructure a nested object, but keep a reference to the nested object

Question

I have a simple use-case, but I think it's not possible with ES6 syntax. I'd like to use object destructuring to retrieve certain known properties from a nested object, but I'd also like a reference to that nested object so that I can pass it along to other functions which may care about other properties.

Here's an example object:

var record = {
    name: "foo",
    metadata: {
        createdDate: "2017-02-19",
        lastModifiedDate: "2018-07-24",
        type: "bar"
    }
};

At a certain point in my code, I'd like to extract some values from the metadata. At the same time, I'd also like to extract the entire metadata object:

let {
    metadata: {
        createdDate
    }
} = record;

if ( date( createdDate ).before( NEW_FEATURE_DATE ) ){
    oldFeature( metadata );
} else {
    newFeature( metadata );
}

Unfortunately, my reference to the metadata property is used purely for destructuring... it's value is undefined (technically, being undefined, it has no value).

I know I can solve this with something like

let {
        metadata: {
            createdDate
        }
    } = record,
    metadata = record.metadata;

or

let {
        metadata
    } = record,
    {
        createdDate
    } = metadata;

... but that seems kind of clumsy, doesn't it? Is there something I'm missing, or is this just a "gap" in the syntax?

With less indentation, the last snippet doesn't look that clumsy actually. — Bergi, Jul 24 '18 at 20:09
@Bergi - Yeah, it's a simple example. Imagine a couple dozen properties. — JDB, Jul 24 '18 at 20:10

score 14 · Accepted Answer · answered Jul 24 '18 at 20:06

14

You could just add another same property without destructuring.

var record = { name: "foo", metadata: { createdDate: "2017-02-19",  lastModifiedDate: "2018-07-24", type: "bar" } };

let { metadata: { createdDate }, metadata } = record;
//    ^^^^^^^^                                        for nested destructuring
//                               ^^^^^^^^             for the value

console.log(metadata);
console.log(createdDate);

answered Jul 24 '18 at 20:06

Nina Scholz

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Awesome! I had hope that I was missing something simple like that. – JDB Jul 24 '18 at 20:11
1

How is this called technically? – Danielo515 Jul 20 '21 at 08:01

Aluan Haddad · Answer 2 · 2018-07-24T20:16:01.223

1

It depends on whether or not you want the nested object to also contain the destructured properties.

If you do then then see Nina Scholz's answer

If you don't then the use the ...rest syntax like so:

let {
    metadata: {
      createdDate,
      ...metadataExceptForCreatedDate
  }
} = record;

edited Jul 24 '18 at 20:16

answered Jul 24 '18 at 20:11

Aluan Haddad

29,886
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Cool. Nina's answer gave me what I need, but this is still very useful information for future reference. – JDB Jul 24 '18 at 20:13
Glad to help. Her answer is also very helpful indeed. – Aluan Haddad Jul 24 '18 at 20:14

Destructure a nested object, but keep a reference to the nested object

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