Automatic Brace Insertion (ABI) is undesirable

TypeScript Version: 3.7-beta

Search Terms: automatic brace insertion ABI syntax error

Code:

function a() {
    "Automatic Brace Insertion! (ABI)";

Expected behavior:

The input is syntactically invalid, so emit should fail regardless of the state of noEmitOnError. Other forms of un-parsable input already do this, e.g. function a(

Actual behavior:

A closing brace is inserted in the emitted JS code.

function a() {
    "Automatic Brace Insertion! (ABI)";
}

This means that users who are not using noEmitOnError are silently having invalid code transformed into valid code. I can confirm this actually happened to someone shipping something to production.

Playground Link: ABI Playground Link

Related Issues: https://github.com/microsoft/TypeScript/issues/34870 , https://github.com/microsoft/TypeScript/issues/34871 , Lazy Twitter report

I would suggest this bug deserves to be fixed based on Design Goal 7 (Preserve runtime behavior of all JavaScript code) and Design Non-Goal 7 (Introduce behaviour that is likely to surprise users).

Tagline: If the braces don't fit, you must not emit.

ABI Playground Link と issue に示されている最小限の TypeScript 入力から始め、リンクされている未終了の関数の例とその emit の動作を比較します。この不正な形式の入力をコンパイラーがどのように処理するかを追跡し、emit が閉じ中かっこを追加せず、実行可能な JavaScript を生成しないことを確認します。