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flimzy opened this issue Aug 23, 2016 · 2 comments
Open

Question: Pointers with js tags #504

flimzy opened this issue Aug 23, 2016 · 2 comments
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bug
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JavaScript intero...

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@flimzy
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flimzy commented Aug 23, 2016 •
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Question:

Is code like the following expected to work?

type X struct {
    *js.Object
    Baz *string `js:"baz"`
}

Of course the reason I ask is that it's not working. But then, I'm not really sure what I would expect it to do. So it seems quite plausible that it's not meant to work. In that case, perhaps a more graceful error message would be my feature request.

At the moment, if I attempt to assign something to X.Baz, then try to retrieve it, I get:

TypeError: Cannot assign to read only property '0' of [object String]

And if I try to read from it before it's initialized, I get:

TypeError: Cannot read property 'constructor' of undefined

Which is the same behavior I get if I attempt to read an uninitialized array with a js tag. But I can initialize the array first, and then things work.

My test code is as follows:

package main

import (
    "fmt"
    "github.com/gopherjs/gopherjs/js"
)

type X struct {
    *js.Object
    Baz *string `js:"baz"`
}

func main() {
    o := js.Global.Get("Object").New()
    x := &X{Object: o}
// Uncomment the following two lines to attempt assignment
//  var oink string = "oink"
//  x.Baz = &oink
    fmt.Printf("x = %s\n", *x.Baz)
}
@dmitshur
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Thanks for reporting this @flimzy.

It's definitely a bug/issue if it's not documented and doesn't work. We should fix it either via documentation and more graceful error message, or by actually implementing it.

I haven't looked to closely at this yet, I plan to do that sometime. At a glance, I suspect we shouldn't try to support this... But I might be missing important details.

Is there a good reason to use pointers rather than values for fields? I'm only asking to understand this issue better.

@dmitshur dmitshur added the bug label Aug 26, 2016
@flimzy
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flimzy commented Aug 26, 2016

The reason I was experimenting with it is that I wanted to use nullable values in a struct. Of course, JS objects already support this in a way that Go does not, as JS distinguishes between undefined and false, undefined and 0, undefined and "", etc.

Ultimately, in my case, I decided that the cleanest solution was a "pure" Go struct (as in, no embedded *js.Object, and no js tags), with a .compile() method which does an intelligent conversion to a map[string]interface{} to be passed to the JS method I was wrapping.

I haven't spent a lot of effort thinking about what I would expect should happen with pointers + js tags, or whether there is a legitimate use case for it. Although I haven't tested, I expect something similar would happen with an interface with a js tag, which might make sense in some cases, such as:

type Foo struct {
    *js.Object
    Error error `js:"error"`
}

@myitcv myitcv mentioned this issue Apr 5, 2017
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