fix: handle async generic arrow functions (#20)

Closes #19

This updates the transform to detect `async` arrow functions that have
type parameters which span over multiple lines. When this is detected
the `(` at the start of the functions parameters is moved to the start
of the type parameters. Avoiding a new line character to appear directly
after the `async` keyword.

Signed-off-by: Ashley Claymore <[email protected]>

Author: acutmore

README.md

@@ -55,7 +55,7 @@ The benefits of this library are:
     -   No [native-addons](https://nodejs.org/api/addons.html)
     -   No [child process](https://nodejs.org/api/child_process.html)
 -   It is small
-    -   Less than 700 lines of code and one dependency ([`TypeScript`](https://www.npmjs.com/package/typescript))
+    -   Around 700 lines of code and one dependency ([`TypeScript`](https://www.npmjs.com/package/typescript))
     -   By doing so little, the code should be relatively easy to maintain
 -   Delegates the parsing to the [official TypeScript parser](https://github.com/microsoft/TypeScript/wiki/Using-the-Compiler-API)
 -   No need for additional SourceMap processing; see ["where are my SourceMaps?"](#where-are-my-sourcemaps)
@@ -159,13 +159,11 @@ statementWithNoSemiColon
 ("not calling above statement");
 ```
 
-### Arrow function return types that introduce a new line
+### Arrow function type annotations that introduce a new line
 
-If the annotation marking the return type of an arrow function introduces a new line before the `=>`, then only replacing it with blank space would be incorrect.
+If the type annotations around an arrow function's parameters introduce a new line then only replacing them with blank space can be incorrect. Therefore, in addition to removing the type annotation, the `(` or `)` surrounding the function parameters may also be moved.
 
-Therefore, in addition to removing the type annotation, the `)` is moved down to the end of the type annotation.
-
-Example input:
+#### Example one - multi-line return type:
 
 <!-- prettier-ignore -->
 ```typescript
@@ -183,6 +181,24 @@ let f = (a        , b
 ) => [a, b];
 ```
 
+#### Example two - `async` with multi-line type arguments:
+
+<!-- prettier-ignore -->
+```typescript
+let f = async <
+    T
+>(v: T) => {};
+```
+
+becomes:
+
+<!-- prettier-ignore -->
+```javascript
+let f = async (
+
+  v   ) => {};
+```
+
 ## Unsupported Syntax
 
 Some parts of TypeScript are not supported because they can't be erased in place due to having runtime semantics. See [unsupported_syntax.md](./docs/unsupported_syntax.md).

package.json

@@ -1,7 +1,7 @@
 {
     "name": "ts-blank-space",
     "description": "A small, fast, pure JavaScript type-stripper that uses the official TypeScript parser.",
-    "version": "0.4.1",
+    "version": "0.4.2",
     "license": "Apache-2.0",
     "homepage": "https://bloomberg.github.io/ts-blank-space",
     "contributors": [

src/blank-string.ts

@@ -1,8 +1,9 @@
 // Copyright 2024 Bloomberg Finance L.P.
 // Distributed under the terms of the Apache 2.0 license.
 
-const FLAG_REPLACE_WITH_CLOSE_PAREN = 1;
-const FLAG_REPLACE_WITH_SEMI = 2;
+const FLAG_REPLACE_WITH_OPEN_PAREN = 1;
+const FLAG_REPLACE_WITH_CLOSE_PAREN = 2;
+const FLAG_REPLACE_WITH_SEMI = 3;
 
 function getSpace(input: string, start: number, end: number): string {
     let out = "";
@@ -31,6 +32,10 @@ export default class BlankString {
         this.__ranges = [];
     }
 
+    blankButStartWithOpenParen(start: number, end: number): void {
+        this.__ranges.push(FLAG_REPLACE_WITH_OPEN_PAREN, start, end);
+    }
+
     blankButEndWithCloseParen(start: number, end: number): void {
         this.__ranges.push(0, start, end - 1);
         this.__ranges.push(FLAG_REPLACE_WITH_CLOSE_PAREN, end - 1, end);
@@ -69,6 +74,9 @@ export default class BlankString {
             } else if (flags === FLAG_REPLACE_WITH_SEMI) {
                 out += ";";
                 rangeStart += 1;
+            } else if (flags === FLAG_REPLACE_WITH_OPEN_PAREN) {
+                out += "(";
+                rangeStart += 1;
             }
 
             previousEnd = rangeEnd;

src/index.ts

@@ -177,7 +177,7 @@ function visitVariableStatement(node: ts.VariableStatement): VisitResult {
 function visitCallOrNewExpression(node: ts.NewExpression | ts.CallExpression): VisitResult {
     visitor(node.expression);
     if (node.typeArguments) {
-        blankGenerics(node, node.typeArguments);
+        blankGenerics(node, node.typeArguments, /*startWithParen*/ false);
     }
     if (node.arguments) {
         const args = node.arguments;
@@ -194,7 +194,7 @@ function visitCallOrNewExpression(node: ts.NewExpression | ts.CallExpression): V
 function visitTaggedTemplate(node: ts.TaggedTemplateExpression): VisitResult {
     visitor(node.tag);
     if (node.typeArguments) {
-        blankGenerics(node, node.typeArguments);
+        blankGenerics(node, node.typeArguments, /*startWithParen*/ false);
     }
     visitor(node.template);
     return VISITED_JS;
@@ -233,7 +233,7 @@ function visitClassLike(node: ts.ClassLikeDeclaration): VisitResult {
 
     // ... <T>
     if (node.typeParameters && node.typeParameters.length) {
-        blankGenerics(node, node.typeParameters);
+        blankGenerics(node, node.typeParameters, /*startWithParen*/ false);
     }
 
     const { heritageClauses } = node;
@@ -260,7 +260,7 @@ function visitClassLike(node: ts.ClassLikeDeclaration): VisitResult {
 function visitExpressionWithTypeArguments(node: ts.ExpressionWithTypeArguments): VisitResult {
     visitor(node.expression);
     if (node.typeArguments) {
-        blankGenerics(node, node.typeArguments);
+        blankGenerics(node, node.typeArguments, /*startWithParen*/ false);
     }
     return VISITED_JS;
 }
@@ -311,6 +311,14 @@ function visitModifiers(modifiers: ArrayLike<ts.ModifierLike>): void {
     }
 }
 
+function isAsync(modifiers: ArrayLike<ts.ModifierLike> | undefined): boolean {
+    if (!modifiers) return false;
+    for (let i = 0; i < modifiers.length; i++) {
+        if (modifiers[i].kind === SK.AsyncKeyword) return true;
+    }
+    return false;
+}
+
 /**
  * prop: T
  */
@@ -394,14 +402,19 @@ function visitFunctionLikeDeclaration(node: ts.FunctionLikeDeclaration, kind: ts
         visitor(node.name);
     }
 
+    let moveOpenParen = false;
     if (node.typeParameters && node.typeParameters.length) {
-        blankGenerics(node, node.typeParameters);
+        moveOpenParen = isAsync(node.modifiers) && spansLines(node.typeParameters.pos, node.typeParameters.end);
+        blankGenerics(node, node.typeParameters, moveOpenParen);
     }
 
     // method?
     node.questionToken && blankExact(node.questionToken);
 
     const params = node.parameters;
+    if (moveOpenParen) {
+        str.blank(params.pos - 1, params.pos);
+    }
     for (let i = 0; i < params.length; i++) {
         const p = params[i];
         if (i === 0 && p.name.getText(ast) === "this") {
@@ -594,10 +607,10 @@ function blankExactAndOptionalTrailingComma(n: ts.Node): void {
 /**
  * `<T1, T2>`
  */
-function blankGenerics(node: ts.Node, arr: ts.NodeArray<ts.Node>): void {
+function blankGenerics(node: ts.Node, arr: ts.NodeArray<ts.Node>, startWithParen: boolean): void {
     const start = arr.pos - 1;
     const end = scanRange(arr.end, node.end, getGreaterThanToken);
-    str.blank(start, end);
+    startWithParen ? str.blankButStartWithOpenParen(start, end) : str.blank(start, end);
 }
 
 function getClosingParenthesisPos(node: ts.NodeArray<ts.ParameterDeclaration>): number {

tests/fixture/cases/async-generic-arrow.ts

@@ -0,0 +1,18 @@
+
+// Simple case
+const a = async<T>(v: T) => {};
+//             ^^^  ^^^
+
+// Hard case - generic spans multiple lines
+const b = async <
+    T
+>/**/(/**/v: T) => {};
+//   ^     ^^^
+
+// Harder case - generic and return type spans multiple lines
+const c = async <
+    T
+>(v: T): Promise<
+// ^^^ ^^^^^^^^^^
+    T
+> => v;