Array.prototype.filter doesn't require callback to return

I dig a bit and found out in #5850 and subsequently in #7779 the signature of filter has been changed not to return a boolean, so that people could return a truthy value in the implementation of the filter.

While I undertand the convenience in terms of ease of use, this as far as I observed puts developers at risk of simple bugs like the following:

const even = [1,2,3].filter(x => {
    x % 2 == 0
})

Notice the missing return statement means the function returns undefined, which translates to false, which filters all the items out. This applies also in the case of noImplicitReturns set to true, since the function return type is any, thus it could be a void function for what the compiler knows.

I believe TypeScript's job should be to put this kind of cases in check actually, so that things like truthy values (which sometimes aren't as obvious as we might think and introduce bugs we don't really understand) can be better taken care of.

One solution to make both parties happy would be to optionally support truthy values (maybe an extra compiler flag? Another truthy type to allow for more relaxed checking?) and reintroduce the original signature of filter.

まず Array.prototype.filter のシグネチャと、issue #5850 および #7779 の履歴を確認します。意図された truthy 値の動作を維持しつつ、要求されたチェックで return 文を省略したコールバックを拒否すべきかどうかを判断します。望ましい型チェックの動作と互換性に関するトレードオフが仕様化され、適切なコンパイラテストでカバーされた時点で issue は完了です。