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useDefineForClassFields and parameter properties: missing error or bad emit #36410

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ajafff opened this issue Jan 24, 2020 · 4 comments · Fixed by #36425
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useDefineForClassFields and parameter properties: missing error or bad emit #36410

ajafff opened this issue Jan 24, 2020 · 4 comments · Fixed by #36425
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Bug A bug in TypeScript Fix Available A PR has been opened for this issue

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@ajafff
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ajafff commented Jan 24, 2020

TypeScript Version: 3.8.0-dev.20200123

Search Terms: useDefineForClassFields parameter

Code

// @useDefineForClassFields: true
// @target: esnext
class C {
  bar = this.foo.bar;
  constructor(private foo: {bar: string}) {}
}

Expected behavior:

Transpiled code works (bar is defined in the constructor as it's done for the downlevel emit) OR used-before-assignment error on this.foo in the initializer of bar.

Actual behavior:

Since the initializer of bar is run before the assignment of foo, there's a runtime error.
This code used to work for ages (of course without native class fields). A lot of code that uses dependency injection looks like this.

Playground Link: https://www.typescriptlang.org/play/?useDefineForClassFields=true&target=99&ts=3.8.0-dev.20200123&ssl=4&ssc=2&pln=1&pc=1#code/MYGwhgzhAEDC0G8BQ1oCMwCdoF5oBcALASwgDoAzAeyrI0wG4VpgqA7CfTAV2HyswAKAA6ZiANzD4AptGpUAXInpLOYtgHMAvgEpEWpFqA

Related Issues:
I thought this was already discussed somewhere with @RyanCavanaugh and @sandersn, but I couldn't find an issue for that.
Found while looking at #36405. That PR would need an exception if parameter properties are involved IF you want to continue supporting the example code.

@sandersn
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Support for [[Define]] so far has given an error whenever code that used to work for [[Set]] is no longer valid. I think that an error with useDefineForClassFields: true is the correct thing here. This error should move to useDefineForClassFields: false at the same time as the other errors from #33401 and #33423.

@sandersn sandersn self-assigned this Jan 24, 2020
@sandersn sandersn added the Bug A bug in TypeScript label Jan 24, 2020
@sandersn sandersn added this to Not started in Rolling Work Tracking Jan 24, 2020
@sandersn
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sandersn commented Jan 24, 2020

The emitted code is also wrong, but in a different way, for targets lower than esnext:

// @useDefineForClassFields: true
// @target: es2019
class C {
    constructor(foo) {
        this.foo = foo;
        Object.defineProperty(this, "foo", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: void 0
        });
        Object.defineProperty(this, "bar", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: this.foo.bar
        });
    }
}

"foo"'s defineProperty should come before this.foo = foo (or should replace it entirely).

Edit: This should not be an error since we're not trying to emit class fields.

@sandersn sandersn moved this from Not started to In Progress in Rolling Work Tracking Jan 24, 2020
@sandersn
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Never mind, the bug I saw must be fixed because it only repros in the playground.

@ajafff
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ajafff commented Jan 24, 2020

@sandersn yeah, you fixed that a while back in #34987
... and in that PR we already talked about the order of parameter properties vs. class field initializers.

@sandersn sandersn added the Fix Available A PR has been opened for this issue label Jan 24, 2020
@sandersn sandersn moved this from In Progress to In Review in Rolling Work Tracking Jan 24, 2020
Rolling Work Tracking automation moved this from In Review to Done Jan 24, 2020
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