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Say I have an object named foo, and I would like to perform the following operation:

foo.bar.baz.qwer.asdf = 5;

The issue is that bar may not exist, but if it does exist, then baz inside of it may not exist, etc.

I thought I would be able to do something like this:

foo?.bar?.baz?.qwer?.asdf = 5, but alas, that sort of chaining only seems to work with retrieving values, not setting values.

The desired behavior is that if something like baz already exists, then that object is just accessed (and its existing members are maintained), but if baz does not exist, then that property is created on the fly.

That is, let's say only foo.bar existed. Then after the above assignment operation, the resulting object would be:

foo {
  bar: {
    baz: {
      qwer: {
        asdf: 5
      }
    }
  }
}

So my question is: can this be done with existing language syntax constructs, or must a helper function be created? If so, does this sort of function have a name that I could search for and reference?

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  • I think you have too check if each property exists unless you want to overwrite them. Commented Jan 7, 2022 at 16:36
  • @about14sheep Yeah, that would work, but it would be too cumbersome, verbose, and error-prone in this case, as I have very deep objects, and need to perform this sort of operation in many different places. Commented Jan 7, 2022 at 16:37
  • so if bar exists, do you not want to keep iterating until you get too asdf? Or are you only concerned with the final property in the chain existing? Commented Jan 7, 2022 at 16:40
  • This is how PHP array assignment works, but you'd have to roll your own function to make it work in Javascript. Commented Jan 7, 2022 at 16:44
  • @about14sheep The latter. The final property must be made, and any intermediate objects along the way should be constructed if necessary. Commented Jan 7, 2022 at 16:46

1 Answer 1

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I have written this:

function setProperty(obj, property, value) {
    const properties = property.split('.');
    const lastProperty = properties.pop();
    for (let prop of properties) {
        if (!obj[prop]) {
            obj[prop] = {};
        }
        obj = obj[prop];
    }
    obj[lastProperty] = value;
}

const startObj = {'a': {}};

setProperty(startObj, 'a.b.c.d.e', 'e is the best letter');
console.log(startObj);

However, it will have unexpected behaviors if one of the intermediate properties exists but isn't an object. Eg test the above function with the starting object {a:{b:2}}

Also, please note that what obj means changes as the function processes. At first, it means the base object, but it goes on to mean each nested object during the loop.

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5 Comments

Thanks, this function looks good, and that unexpected behavior should be fine, I think, as that would just be a bug with the application anyway, which should cause a crash and investigation. Thanks.
I found a guy that gave functionally equivalent code to a similar question before: stackoverflow.com/questions/18936915/…
Ah, there it is. I was looking for that thread but my Googlefu was failing me. Good find
I think my code is better though ;)
Yeah, I agree. Also, lodash seems to have a set function, which I may use, since this project has already imported lodash anyway. Anyways, thanks again!

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