Unexpected "in" error in esbuild but not Typescript playground

[breunigs]

Using esbuild 0.17.14 to compile this snippet:

const LAYERS: { [keyof in "bar"]?: boolean } = {
    bar: true,
};

results in

✘ [ERROR] Unexpected "in"

    foo.ts:1:23:
      1 │ const LAYERS: { [keyof in "bar"]?: boolean } = {
        ╵                        ~~

1 error

whereas the TS playground seems not to mind:
https://www.typescriptlang.org/play?#code/MYewdgzgLgBAMgQQJoFEBKBlAXDA3jAbQGsBTATxADMYBLMGAIgCMBDAJwYF0B+HJkEABsSLegF8YAXjwAoGPJis2OKGwCuJADQyxAbhmhIQkgDpBIAOYAKRKkwBKIA

This approach is used here: https://github.com/maplibre/maplibre-gl-js/blob/3157c340e5573699e46ce4b3fc17e9e670442e1a/src/render/render_to_texture.ts#L13-L19 if a non-toy example is desired.

I am not familiar enough with the JS/TS ecosystem to judge what's wrong here, I am hoping you know.

[hyrious]

The keyof in this example is interpreted as a type name instead of a keyword. That is to say it is effectively the same as

const LAYERS: { [P in "bar"]?: boolean }