A question regarding spy and CaughtException

[kubk]

If you run this codesandbox you'll notice that there is the following spy report:

{
observableKind: "computed"
debugObjectName: "ObservableObject@1.isEven"
object: Object
type: "update"
oldValue: CaughtException
newValue: true
}

Notice the oldValue: CaughtException. Technically the spy report is correct because the initial value of a computed is CaughtException:

mobx/packages/mobx/src/core/computedvalue.ts

Line 91 in e60b36c

protected value_: T | undefined | CaughtException = new CaughtException(null)

But from a library consumer point of view the initial value of isEven computed is true. I've noticed it while developing a logging library for Mobx. I keep getting log messages like this:

I'd like to filter such messages out. But neither CaughtException nor isCaughtException is a part of public Mobx API. What'd you suggest me to solve the issue?

[urugator]

If we export isCaughtException won't there be some problem with differentiating initial value and actual error?
Do you need isCaughtException in other places?

Btw ES newly supports cause on standard Error maybe it would be nice to make use of it.

[kubk]

@urugator Thanks, I decided to use value?.constructor?.name === 'CaughtException' check to determine if value is CaughtException. So there is no need to import isCaughtException.

won't there be some problem with differentiating initial value and actual error?

It looks I don't need to differentiate them. I'll just hide all the logs related to CaughtException

[urugator]

Occured to me as well, but then I thought it would not work when minified.... ?

[mweststrate]

Yeah depends on minifier settings, some configs will minify the constructor name.

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[kubk]

@mweststrate @urugator Thank you, you were right. I've built mobx locally and see that CaughtException doesn't exist in the prod build, because the property mangling is enabled:

mobx/packages/mobx/tsdx.config.js

Line 25 in 56bfb67

mangle: {

Btw ES newly supports cause on standard Error maybe it would be nice to make use of it.

Wouldn't it increase the bundle size? Now it makes sense for me to make isCaughtException a part of public Mobx API, it won't affect the bundle size. I'd be happy to hear your opinions.

[kubk]

@urugator That is a lot of work 😃 I think if we change the initial value of computed to Symbol it'd solve the initial issue. I'd skip logging Symbols because they can't be observable. Do you think changing CaughtException to Symbol('initialValue') may bring some performance optimizations?

[urugator]

I'd skip logging Symbols because they can't be observable.

I don't follow. oldValue can be whatever returned from computed ... the current CaughtException isn't observable as well.

Do you think changing CaughtException to Symbol('initialValue') may bring some performance optimizations?

In principle yes, in practice most likely no. Not sure it there is some technical reason for initial values being unique per computed instance. Failing tests should tell if that's the case I guess.

I am fine with changing initial value to symbol and exporting it, I just don't want to expose CaughtException in it's current form.

[urugator]

Just occured to me maybe you could also solve with with a little "hack". On the first computed spy event, capture the initial value somewhere and then for all other computed spy events:
Object.getPrototypeOf(initialValue) === Object.getPrototypeOf(oldValue)) && oldValue.cause === null. You can create a dummy computed to force the first spy event with oldValue being CaughtException.

[mweststrate]

Symbols are definitely better than Exceptions here indeed (blame me). If symbols cause trouble, you could also reserve a constant unique, frozen object for it, something like const UninitializedMarker = { MobxUninitializedValue: true }.

[mweststrate]

It'd be fine to expose that object as well, as long as clearly named and sealed, to make it possible to reflect on it.