Add convert const to let

src/compiler/diagnosticMessages.json

@@ -5140,6 +5140,10 @@
         "category": "Message",
         "code": 95092
     },
+    "Convert 'const' to 'let'": {
+        "category": "Message",
+        "code": 95093
+    },
 
     "No value exists in scope for the shorthand property '{0}'. Either declare one or provide an initializer.": {
         "category": "Error",

src/services/codefixes/convertConstToLet.ts

@@ -0,0 +1,32 @@
+/* @internal */
+namespace ts.codefix {
+    const fixId = "fixConvertConstToLet";
+    const errorCodes = [Diagnostics.Cannot_assign_to_0_because_it_is_a_constant.code];
+
+    registerCodeFix({
+        errorCodes,
+        getCodeActions: context => {
+            const { sourceFile, span, program } = context;
+            const variableStatement = getVariableStatement(sourceFile, span.start, program);
+            const changes = textChanges.ChangeTracker.with(context, t => doChange(t, sourceFile, variableStatement));
+            return [createCodeFixAction(fixId, changes, Diagnostics.Convert_const_to_let, fixId, Diagnostics.Convert_const_to_let)];
+        },
+        fixIds: [fixId]
+    });
+
+    function getVariableStatement(sourceFile: SourceFile, pos: number, program: Program) {
+        const token = getTokenAtPosition(sourceFile, pos);
+        const checker = program.getTypeChecker();
+        const symbol = checker.getSymbolAtLocation(token);
+        if (symbol) {
+            return symbol.valueDeclaration.parent.parent as VariableStatement;
+        }
+    }
+    function doChange(changes: textChanges.ChangeTracker, sourceFile: SourceFile, variableStatement?: VariableStatement) {
+        if (!variableStatement) {
+            return;
+        }
+        const start = variableStatement.getStart();
+        changes.replaceRangeWithText(sourceFile, { pos: start, end: start + 5 }, "let");
+    }
+}

src/services/tsconfig.json

@@ -83,6 +83,7 @@
         "codefixes/fixAddModuleReferTypeMissingTypeof.ts",
         "codefixes/convertToMappedObjectType.ts",
         "codefixes/removeUnnecessaryAwait.ts",
+        "codefixes/convertConstToLet.ts",
         "refactors/convertExport.ts",
         "refactors/convertImport.ts",
         "refactors/extractSymbol.ts",

tests/cases/fourslash/codeFixConstToLet.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts' />
+
+//// const x = 42;
+//// x = 75;
+
+verify.codeFix({
+  description: "Convert 'const' to 'let'",
+  index: 0,
+  newFileContent: 
+`let x = 42;
+x = 75;`
+});