That's right, this
Did you notice in the example above we not only copied the object, but also the nested array, and even the Date object?
And all works precisely as expected:
structuredClone can not only do the above, but additionally:
ImageData, and many more
So for example, this madness would even work as expected:
It is important to note we are talking about a deep copy. If you just need to do a shallow copy, aka a copy that does not copy nested objects or arrays, then we can just do an object spread:
Or even one of these, if you prefer
But as soon as we have nested items, we run into trouble:
As you can see, we did not make a full copy of this object.
The nested date and array are still a shared reference between both, which can cause us major issues if we want to edit those thinking we are only updating the copied calendar event object.
Ah yes, this trick. It is actually a great one, and is surprisingly performant, but has some shortcomings that
Take this as an example:
If we log
problematicCopy, we would get:
That’s not what we wanted!
date is supposed to be a
Date object, not a string.
This happened because
JSON.stringify can only handle basic objects, arrays, and primitives. Any other type can be handled in hard to predict ways. For instance, Dates are converted to a string. But a
Set is simply converted to
JSON.stringify even completely ignores certain things, like
undefined or functions.
For instance, if we copied our
kitchenSink example with this method:
We would get:
Oh yeah, and we had to remove the circular reference we originally had for this, as
JSON.stringify simply throws errors if it encounters one of those.
So while this method can be great if our requirements fit what it can do, there is a lot that we can do with
structuredClone (aka everything above that we failed to do here) that this method cannot.
To date, Lodash’s
cloneDeep function has been a very common solution to this problem.
And this does, in fact, work as expected:
But, there is just one caveat here. According to the Import Cost extension in my IDE, that prints the kb cost of anything I import, this one function comes in at a whole 17.4kb minified (5.3kb gzipped):
And that assumes you import just that function. If you instead import the more common way, not realizing that tree shaking doesn’t always work the way you hoped, you could accidentally import up to 25kb just for this one function 😱
While that will not be the end of the world to anyone, it’s simply not necessary in our case, not when browsers already have
structuredClone built in.
They will a throw a
Also throws a
As well as similar metadata-like features are not cloned.
For instance, with a getter, the resulting value is cloned, but not the getter function itself (or any other property metadata):
The prototype chain is not walked or duplicated. So if you clone an instance of
MyClass, the cloned object will no longer be known to be an instance of this class (but all valid properties of this class will be cloned)
More simply put, anything not in the below list cannot be cloned:
Error types (those specifically listed below),
Object but only plain objects (e.g. from object literals), Primitive types, except
And here is the best part -
structuredClone is supported in all major browsers, and even Node.js and Deno.
Just note the caveat with Web Workers having more limited support:
It’s been a long time coming, but we finally now have
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