Javascript nullish assignment of each object member

Question

Assuming I have the following two objects

foo = {
  a: 10
  b: 'hello'
  c: 'world'
}

bar = {
  a:5
  b: null
  c: null
  d: "This is not in foo"
}

I would like to have an operation which would do the equivalent of below operation but without having to specify it for each member.

  bar.a ??= foo.a
  bar.b ??= foo.b
  bar.c ??= foo.c

  console.log(bar) // {a:5, b:'hello', c:'world', d:'This is not in foo'

Essentially: For each member of bar, if it is nullish take the value in foo. Leave all members which exist in foo but not in bar in peace

How would I go about this? I have tried to search a solution using destructuring in some way but with no success...

Barmar · Accepted Answer · 2022-11-16 17:35:35Z

2

Loop over the keys in foo then perform the assignment operation in the loop.

Object.keys(bar).forEach(k => bar[k] ??= foo[k]);

edited Nov 16, 2022 at 17:35

answered Nov 16, 2022 at 17:12

Barmar

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6 Comments

sev Over a year ago

derpischer in their solution mentioned that if bar has some undefined properties, they would not be overwritten, is this also the case in your spread syntax solution?

Barmar Over a year ago

No, it only replaces properties that don't exist at all. I've removed that from the answer.

sev Over a year ago

Should it not be Object.keys(bar)...?

Barmar Over a year ago

Yes, when I started writing the answer I had the relationship backwards, and didn't fix it properly.

sev Over a year ago

I am actually using typescript and am getting some problems when trying to implement your solution. I have posted a related question about it here: stackoverflow.com/questions/74465046/…

|

derpirscher · Accepted Answer · 2022-11-16 19:18:05Z

0

If you can create a new object instead of manipulating bar you could do as follows

let newbar = Object.assign({}, foo, bar);

Ie, in the Object.assign it will first assign all properties from foo and then all properties from bar, so properties not existing in bar will be taken from foo and properties existing in bar will be taken from bar.

Be aware, that if bar has an undefined property, this will overwrite the value from foo. So

let bar = { a: undefined }
let foo = { a: 3 }
let combined = Object.assign({}, foo, bar);  
// will result in { a: undefined }

Also be aware, that this will add any property, that exists in foo to the result. So

 let bar = { a: 3 }
 let foo = { b: 4 }
 let combined = Object.assign({}, foo, bar);  
 // will result in { a: 3, b: 4 }

Not sure if this fits your requirement. If not, go with Barmar's solution.

edited Nov 16, 2022 at 19:18

answered Nov 16, 2022 at 17:19

derpirscher

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2 Comments

sev Over a year ago

How could I avoid the undefined problem?

AKd Over a year ago

-1, this does not solve the problem: “For each member of bar, if it is nullish take the value in foo”. Your proposal keep nullish values from bar into the resulting object.

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