TC39 General
2023-04-20
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15:52
<Jack Works>
why class field initialization order is all field initializers runs first, then the constructor?
15:53
<Jack Works>
i thought it is more intuitive to run in source text order?
15:54
<Jack Works>

like this:

class T extends Q {
    field = 2
    constructor() {
        1
        super()
        3
    }
    field2 = 4
}
15:55
<Jack Works>
spec order is 1-2-4-3
16:06
<bakkot>
generally it's more useful to have fields set up before the constructor runs
16:06
<Jack Works>
is there some design principles behind this?
16:06
<Jack Works>
and is it related to decorators?
16:32
<ljharb>
if it ran in source text order then the this it installs field2 on might not be the actual instance, since the constructor could have returned any object
16:33
<ljharb>
it's pretty important for private fields imo, at least, that all of them are installed on the same object, so you can never observe a partially "set up" instance
16:33
<Jack Works>
source text order 1-2-3-4 also installed on the correct this value IMO
16:35
<ljharb>
if line 6.5 did return {} which object would get field2?
16:35
<Jack Works>
oh...
16:36
<Jack Works>
ok I understand now