Merge pull request microsoft#18926 from amcasey/ExtractConstantArrow

Disallow constant extraction into expression-bodied arrow functions

Author: amcasey

src/harness/unittests/extractConstants.ts

@@ -214,6 +214,14 @@ const x = [#|2 + 1|];
 /* About x */
 const x = [#|2 + 1|];
         `);
+
+        testExtractConstant("extractConstant_ArrowFunction_Block", `
+const f = () => {
+    return [#|2 + 1|];
+};`);
+
+        testExtractConstant("extractConstant_ArrowFunction_Expression",
+            `const f = () => [#|2 + 1|];`);
     });
 
     function testExtractConstant(caption: string, text: string) {

src/services/refactors/extractSymbol.ts

@@ -142,6 +142,7 @@ namespace ts.refactor.extractSymbol {
         export const CannotAccessVariablesFromNestedScopes = createMessage("Cannot access variables from nested scopes");
         export const CannotExtractToOtherFunctionLike = createMessage("Cannot extract method to a function-like scope that is not a function");
         export const CannotExtractToJSClass = createMessage("Cannot extract constant to a class scope in JS");
+        export const CannotExtractToExpressionArrowFunction = createMessage("Cannot extract constant to an arrow function without a block");
     }
 
     enum RangeFacts {
@@ -1299,6 +1300,10 @@ namespace ts.refactor.extractSymbol {
             if (isClassLike(scope) && isInJavaScriptFile(scope)) {
                 constantErrors.push(createDiagnosticForNode(scope, Messages.CannotExtractToJSClass));
             }
+            if (isArrowFunction(scope) && !isBlock(scope.body)) {
+                // TODO (https://github.com/Microsoft/TypeScript/issues/18924): allow this
+                constantErrors.push(createDiagnosticForNode(scope, Messages.CannotExtractToExpressionArrowFunction));
+            }
             constantErrorsPerScope.push(constantErrors);
         }
 

tests/baselines/reference/extractConstant/extractConstant_ArrowFunction_Block.js

@@ -0,0 +1,18 @@
+// ==ORIGINAL==
+
+const f = () => {
+    return 2 + 1;
+};
+// ==SCOPE::Extract to constant in enclosing scope==
+
+const f = () => {
+    const newLocal = 2 + 1;
+
+    return /*RENAME*/newLocal;
+};
+// ==SCOPE::Extract to constant in global scope==
+const newLocal = 2 + 1;
+
+const f = () => {
+    return /*RENAME*/newLocal;
+};

tests/baselines/reference/extractConstant/extractConstant_ArrowFunction_Block.ts

@@ -0,0 +1,18 @@
+// ==ORIGINAL==
+
+const f = () => {
+    return 2 + 1;
+};
+// ==SCOPE::Extract to constant in enclosing scope==
+
+const f = () => {
+    const newLocal = 2 + 1;
+
+    return /*RENAME*/newLocal;
+};
+// ==SCOPE::Extract to constant in global scope==
+const newLocal = 2 + 1;
+
+const f = () => {
+    return /*RENAME*/newLocal;
+};

tests/baselines/reference/extractConstant/extractConstant_ArrowFunction_Expression.js

@@ -0,0 +1,6 @@
+// ==ORIGINAL==
+const f = () => 2 + 1;
+// ==SCOPE::Extract to constant in global scope==
+const newLocal = 2 + 1;
+
+const f = () => /*RENAME*/newLocal;

tests/baselines/reference/extractConstant/extractConstant_ArrowFunction_Expression.ts

@@ -0,0 +1,6 @@
+// ==ORIGINAL==
+const f = () => 2 + 1;
+// ==SCOPE::Extract to constant in global scope==
+const newLocal = 2 + 1;
+
+const f = () => /*RENAME*/newLocal;