Support 'this' in inferred method bodies

src/compiler/checker.ts

@@ -6897,6 +6897,23 @@ namespace ts {
                 const symbol = getSymbolOfNode(container.parent);
                 return container.flags & NodeFlags.Static ? getTypeOfSymbol(symbol) : (<InterfaceType>getDeclaredTypeOfSymbol(symbol)).thisType;
             }
+
+            // If this is a function in a JS file, it might be a class method. Check if it's the RHS
+            // of a x.prototype.y = function [name]() { .... }
+            if (isInJavaScriptFile(node) && container.kind === SyntaxKind.FunctionExpression) {
+                if (getSpecialPropertyAssignmentKind(container.parent) === SpecialPropertyAssignmentKind.PrototypeProperty) {
+                    // Get the 'x' of 'x.prototype.y = f' (here, 'f' is 'container')
+                    const className = (((container.parent as BinaryExpression)   // x.protoype.y = f
+                                        .left as PropertyAccessExpression)       // x.prototype.y
+                                        .expression as PropertyAccessExpression) // x.prototype
+                                        .expression;                             // x
+                    const classSymbol = checkExpression(className).symbol;
+                    if (classSymbol.members) {
+                        return createAnonymousType(undefined, classSymbol.members, emptyArray, emptyArray, /*stringIndexType*/ undefined, /*numberIndexType*/ undefined);
+                    }
+                }
+            }
+
             return anyType;
         }
 

tests/cases/fourslash/javaScriptPrototype3.ts

@@ -0,0 +1,20 @@
+///<reference path="fourslash.ts" />
+
+// Inside an inferred method body, the type of 'this' is the class type
+
+// @allowNonTsExtensions: true
+// @Filename: myMod.js
+//// function myCtor(x) {
+////     this.qua = 10;
+//// }
+//// myCtor.prototype.foo = function() { return this/**/; };
+//// myCtor.prototype.bar = function() { return '' };
+//// 
+
+goTo.marker();
+edit.insert('.');
+
+// Check members of the function
+verify.completionListContains('foo', undefined, undefined, 'property');
+verify.completionListContains('bar', undefined, undefined, 'property');
+verify.completionListContains('qua', undefined, undefined, 'property');

tests/cases/fourslash/javaScriptPrototype5.ts

@@ -0,0 +1,19 @@
+///<reference path="fourslash.ts" />
+
+// No prototype assignments are needed to enable class inference
+
+// @allowNonTsExtensions: true
+// @Filename: myMod.js
+//// function myCtor() {
+////     this.foo = 'hello';
+////     this.bar = 10;
+//// }
+//// let x = new myCtor();
+//// x/**/
+
+goTo.marker();
+edit.insert('.');
+
+// Check members of the function
+verify.completionListContains('foo', undefined, undefined, 'property');
+verify.completionListContains('bar', undefined, undefined, 'property');