Issue #27301: Fixed crash when converting function to async (#27396)

Author: dhruvrajvanshi

src/services/codefixes/convertToAsyncFunction.ts

@@ -451,7 +451,11 @@ namespace ts.codefix {
                     }
 
                     return shouldReturn ? refactoredStmts.map(s => getSynthesizedDeepClone(s)) :
-                        removeReturns(refactoredStmts, prevArgName!.identifier, transformer, seenReturnStatement);
+                        removeReturns(
+                            refactoredStmts,
+                            prevArgName === undefined ? undefined : prevArgName.identifier,
+                            transformer,
+                            seenReturnStatement);
                 }
                 else {
                     const innerRetStmts = getReturnStatementsWithPromiseHandlers(createReturn(funcBody));
@@ -491,14 +495,19 @@ namespace ts.codefix {
     }
 
 
-    function removeReturns(stmts: ReadonlyArray<Statement>, prevArgName: Identifier, transformer: Transformer, seenReturnStatement: boolean): ReadonlyArray<Statement> {
+    function removeReturns(stmts: ReadonlyArray<Statement>, prevArgName: Identifier | undefined, transformer: Transformer, seenReturnStatement: boolean): ReadonlyArray<Statement> {
         const ret: Statement[] = [];
         for (const stmt of stmts) {
             if (isReturnStatement(stmt)) {
                 if (stmt.expression) {
                     const possiblyAwaitedExpression = isPromiseReturningExpression(stmt.expression, transformer.checker) ? createAwait(stmt.expression) : stmt.expression;
-                    ret.push(createVariableStatement(/*modifiers*/ undefined,
-                        (createVariableDeclarationList([createVariableDeclaration(prevArgName, /*type*/ undefined, possiblyAwaitedExpression)], getFlagOfIdentifier(prevArgName, transformer.constIdentifiers)))));
+                    if (prevArgName === undefined) {
+                        ret.push(createExpressionStatement(possiblyAwaitedExpression));
+                    }
+                    else {
+                        ret.push(createVariableStatement(/*modifiers*/ undefined,
+                            (createVariableDeclarationList([createVariableDeclaration(prevArgName, /*type*/ undefined, possiblyAwaitedExpression)], getFlagOfIdentifier(prevArgName, transformer.constIdentifiers)))));
+                    }
                 }
             }
             else {
@@ -507,7 +516,7 @@ namespace ts.codefix {
         }
 
         // if block has no return statement, need to define prevArgName as undefined to prevent undeclared variables
-        if (!seenReturnStatement) {
+        if (!seenReturnStatement && prevArgName !== undefined) {
             ret.push(createVariableStatement(/*modifiers*/ undefined,
                 (createVariableDeclarationList([createVariableDeclaration(prevArgName, /*type*/ undefined, createIdentifier("undefined"))], getFlagOfIdentifier(prevArgName, transformer.constIdentifiers)))));
         }

src/testRunner/unittests/convertToAsyncFunction.ts

@@ -1241,6 +1241,19 @@ _testConvertToAsyncFunction("convertToAsyncFunction_nestedPromises", `
 function [#|f|]() {
     return fetch('https://typescriptlang.org').then(x => Promise.resolve(3).then(y => Promise.resolve(x.statusText.length + y)));
 }
+`);
+_testConvertToAsyncFunction("convertToAsyncFunction_noArgs", `
+function delay(millis: number): Promise<void> {
+    throw "no"
+}
+
+function [#|main2|]() {
+    console.log("Please wait. Loading.");
+    return delay(500)
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+}
 `);
     });
 
@@ -1251,4 +1264,4 @@ function [#|f|]() {
     function _testConvertToAsyncFunctionFailed(caption: string, text: string) {
         testConvertToAsyncFunction(caption, text, "convertToAsyncFunction", /*includeLib*/ true, /*expectFailure*/ true);
     }
-}
+}

tests/baselines/reference/convertToAsyncFunction/convertToAsyncFunction_noArgs.js

@@ -0,0 +1,30 @@
+// ==ORIGINAL==
+
+function delay(millis) {
+    throw "no"
+}
+
+function /*[#|*/main2/*|]*/() {
+    console.log("Please wait. Loading.");
+    return delay(500)
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+}
+
+// ==ASYNC FUNCTION::Convert to async function==
+
+function delay(millis) {
+    throw "no"
+}
+
+async function main2() {
+    console.log("Please wait. Loading.");
+    await delay(500);
+    console.log(".");
+    await delay(500);
+    console.log(".");
+    await delay(500);
+    console.log(".");
+    return delay(500);
+}

tests/baselines/reference/convertToAsyncFunction/convertToAsyncFunction_noArgs.ts

@@ -0,0 +1,30 @@
+// ==ORIGINAL==
+
+function delay(millis: number): Promise<void> {
+    throw "no"
+}
+
+function /*[#|*/main2/*|]*/() {
+    console.log("Please wait. Loading.");
+    return delay(500)
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+        .then(() => { console.log("."); return delay(500); })
+}
+
+// ==ASYNC FUNCTION::Convert to async function==
+
+function delay(millis: number): Promise<void> {
+    throw "no"
+}
+
+async function main2() {
+    console.log("Please wait. Loading.");
+    await delay(500);
+    console.log(".");
+    await delay(500);
+    console.log(".");
+    await delay(500);
+    console.log(".");
+    return delay(500);
+}