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Arrays in JS objects don't seem to work as I'd expect #460

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theclapp opened this issue May 13, 2016 · 3 comments
Closed

Arrays in JS objects don't seem to work as I'd expect #460

theclapp opened this issue May 13, 2016 · 3 comments

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@theclapp
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o := struct {
    *js.Object
    a []int `js:"a"`
    b js.M  `js:"b"`
}{Object: js.Global.Get("Object").New()}
o.a = []int{1}
o.b = js.M{"x": "x"}
fmt.Printf("%v ", o.a[0])
o.a[0] = 2 // <== this doesn't work
fmt.Printf("%v ", o.a[0])

o.Get("a").SetIndex(0, 3) // <== this works but is awkward
fmt.Printf("%v\n", o.a[0])

fmt.Printf("%v ", o.b["x"])
o.b["x"] = "y" // <== this doesn't work either
fmt.Printf("%v ", o.b["x"])

o.Get("b").Set("x", "z") // <== works but awkward
fmt.Printf("%v ", o.b["x"])

result:
1 1 3
x x z

Expected:
1 2 3
x y z

Playground

As near as I can tell, the array "a" gets $internalize()'ed into a whole new array, which is assigned to and then thrown away. Similarly, "b" gets $internalized()'ed into a whole new object, which is assigned to and then thrown away.

Should one just not put slices or maps in JS objects?
@theclapp theclapp changed the title Arrays in JS objects don't seem to work correctly Arrays in JS objects don't seem to work as I'd expect May 13, 2016
@thanhps42
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Me too, slices ad maps in gopherjs make me confused

@theclapp
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Putting "non-scalar" types (like slices, arrays, and maps) in JS-structs doesn't work, for the reason given in my initial comment (an internalized array is assigned to and then thrown away). This is the best I've been able to come up with:

o := struct {
	*js.Object
	a *js.Object `js:"a"` // used as an array
	b *js.Object `js:"b"` // used as a map
}{Object: js.Global.Get("Object").New()}

println("At initialization time:")
o.a = js.Global.Get("Array").New(0, 1, 2)
o.b = js.Global.Get("Object").New(js.M{"x": "y"})
println(o.a) // => [ 0, 1, 2 ]
println(o.b) // => { x: 'y' }

println("\nAfter being modified:")
o.a.SetIndex(2, 3)
o.b.Set("x", "z")
println(o.a) // => [ 0, 1, 3 ]
println(o.b) // => { x: 'z' }

transpiles to

// Printing elided ...
o = new structType.ptr(new ($global.Object)(), null, null);
o.Object.a = new ($global.Array)(0, 1, 2);
o.Object.b = new ($global.Object)($externalize($makeMap($String.keyFor, [{ k: "x", v: new $String("y") }]), js.M));
o.Object.a[2] = 3;
o.Object.b.x = $externalize("z", $String);

On the plus side, this bit of ickyness o.a.SetIndex(2, 3) maps to just this o.Object.a[2] = 3, so at least on the JS side it's fairly clean.

@theclapp
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Added a link to this issue to the Tips and Gotchas wiki page.

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@theclapp @thanhps42