diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 6bd0c0409d435..e6a794e866ac6 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -15809,30 +15809,27 @@ namespace ts {
         }
 
         function tryGetThisTypeAt(node: Node, container = getThisContainer(node, /*includeArrowFunctions*/ false)): Type | undefined {
+            const isInJS = isInJSFile(node);
             if (isFunctionLike(container) &&
                 (!isInParameterInitializerBeforeContainingFunction(node) || getThisParameter(container))) {
                 // Note: a parameter initializer should refer to class-this unless function-this is explicitly annotated.
-
                 // 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 (container.kind === SyntaxKind.FunctionExpression &&
-                    container.parent.kind === SyntaxKind.BinaryExpression &&
-                    getAssignmentDeclarationKind(container.parent as BinaryExpression) === AssignmentDeclarationKind.PrototypeProperty) {
-                    // Get the 'x' of 'x.prototype.y = f' (here, 'f' is 'container')
-                    const className = (((container.parent as BinaryExpression)   // x.prototype.y = f
-                        .left as PropertyAccessExpression)       // x.prototype.y
-                        .expression as PropertyAccessExpression) // x.prototype
-                        .expression;                             // x
+                const className = getClassNameFromPrototypeMethod(container);
+                if (isInJS && className) {
                     const classSymbol = checkExpression(className).symbol;
                     if (classSymbol && classSymbol.members && (classSymbol.flags & SymbolFlags.Function)) {
-                        return getFlowTypeOfReference(node, getInferredClassType(classSymbol));
+                        const classType = getJavascriptClassType(classSymbol);
+                        if (classType) {
+                            return getFlowTypeOfReference(node, classType);
+                        }
                     }
                 }
                 // Check if it's a constructor definition, can be either a variable decl or function decl
                 // i.e.
                 //   * /** @constructor */ function [name]() { ... }
                 //   * /** @constructor */ var x = function() { ... }
-                else if ((container.kind === SyntaxKind.FunctionExpression || container.kind === SyntaxKind.FunctionDeclaration) &&
+                else if (isInJS &&
+                         (container.kind === SyntaxKind.FunctionExpression || container.kind === SyntaxKind.FunctionDeclaration) &&
                          getJSDocClassTag(container)) {
                     const classType = getJavascriptClassType(container.symbol);
                     if (classType) {
@@ -15852,7 +15849,7 @@ namespace ts {
                 return getFlowTypeOfReference(node, type);
             }
 
-            if (isInJSFile(node)) {
+            if (isInJS) {
                 const type = getTypeForThisExpressionFromJSDoc(container);
                 if (type && type !== errorType) {
                     return getFlowTypeOfReference(node, type);
@@ -15860,6 +15857,34 @@ namespace ts {
             }
         }
 
+        function getClassNameFromPrototypeMethod(container: Node) {
+            // Check if it's the RHS of a x.prototype.y = function [name]() { .... }
+            if (container.kind === SyntaxKind.FunctionExpression &&
+                isBinaryExpression(container.parent) &&
+                getAssignmentDeclarationKind(container.parent) === AssignmentDeclarationKind.PrototypeProperty) {
+                // Get the 'x' of 'x.prototype.y = container'
+                return ((container.parent   // x.prototype.y = container
+                    .left as PropertyAccessExpression)       // x.prototype.y
+                    .expression as PropertyAccessExpression) // x.prototype
+                    .expression;                             // x
+            }
+            // x.prototype = { method() { } }
+            else if (container.kind === SyntaxKind.MethodDeclaration &&
+                container.parent.kind === SyntaxKind.ObjectLiteralExpression &&
+                isBinaryExpression(container.parent.parent) &&
+                getAssignmentDeclarationKind(container.parent.parent) === AssignmentDeclarationKind.Prototype) {
+                return (container.parent.parent.left as PropertyAccessExpression).expression;
+            }
+            // x.prototype = { method: function() { } }
+            else if (container.kind === SyntaxKind.FunctionExpression &&
+                container.parent.kind === SyntaxKind.PropertyAssignment &&
+                container.parent.parent.kind === SyntaxKind.ObjectLiteralExpression &&
+                isBinaryExpression(container.parent.parent.parent) &&
+                getAssignmentDeclarationKind(container.parent.parent.parent) === AssignmentDeclarationKind.Prototype) {
+                return (container.parent.parent.parent.left as PropertyAccessExpression).expression;
+            }
+        }
+
         function getTypeForThisExpressionFromJSDoc(node: Node) {
             const jsdocType = getJSDocType(node);
             if (jsdocType && jsdocType.kind === SyntaxKind.JSDocFunctionType) {
diff --git a/tests/baselines/reference/jsdocTemplateTag5.errors.txt b/tests/baselines/reference/jsdocTemplateTag5.errors.txt
deleted file mode 100644
index e24cd0926b6d8..0000000000000
--- a/tests/baselines/reference/jsdocTemplateTag5.errors.txt
+++ /dev/null
@@ -1,77 +0,0 @@
-tests/cases/conformance/jsdoc/a.js(18,21): error TS2339: Property '_map' does not exist on type '{ get(key: K): V; }'.
-tests/cases/conformance/jsdoc/a.js(39,21): error TS2339: Property '_map' does not exist on type '{ get: (key: K) => V; }'.
-tests/cases/conformance/jsdoc/a.js(61,21): error TS2339: Property '_map' does not exist on type '{ get(key: K): V; }'.
-
-
-==== tests/cases/conformance/jsdoc/a.js (3 errors) ====
-    /**
-     * Should work for function declarations
-     * @constructor
-     * @template {string} K
-     * @template V
-     */
-    function Multimap() {
-        /** @type {Object<string, V>} TODO: Remove the prototype from the fresh object */
-        this._map = {};
-    };
-    
-    Multimap.prototype = {
-        /**
-         * @param {K} key the key ok
-         * @returns {V} the value ok
-         */
-        get(key) {
-            return this._map[key + ''];
-                        ~~~~
-!!! error TS2339: Property '_map' does not exist on type '{ get(key: K): V; }'.
-        }
-    }
-    
-    /**
-     * Should work for initialisers too
-     * @constructor
-     * @template {string} K
-     * @template V
-     */
-    var Multimap2 = function() {
-        /** @type {Object<string, V>} TODO: Remove the prototype from the fresh object */
-        this._map = {};
-    };
-    
-    Multimap2.prototype = {
-        /**
-         * @param {K} key the key ok
-         * @returns {V} the value ok
-         */
-        get: function(key) {
-            return this._map[key + ''];
-                        ~~~~
-!!! error TS2339: Property '_map' does not exist on type '{ get: (key: K) => V; }'.
-        }
-    }
-    
-    var Ns = {};
-    /**
-     * Should work for expando-namespaced initialisers too
-     * @constructor
-     * @template {string} K
-     * @template V
-     */
-    Ns.Multimap3 = function() {
-        /** @type {Object<string, V>} TODO: Remove the prototype from the fresh object */
-        this._map = {};
-    };
-    
-    Ns.Multimap3.prototype = {
-        /**
-         * @param {K} key the key ok
-         * @returns {V} the value ok
-         */
-        get(key) {
-            return this._map[key + ''];
-                        ~~~~
-!!! error TS2339: Property '_map' does not exist on type '{ get(key: K): V; }'.
-        }
-    }
-    
-    
\ No newline at end of file
diff --git a/tests/baselines/reference/jsdocTemplateTag5.symbols b/tests/baselines/reference/jsdocTemplateTag5.symbols
index 249e92ab257f6..aa1b023f339cc 100644
--- a/tests/baselines/reference/jsdocTemplateTag5.symbols
+++ b/tests/baselines/reference/jsdocTemplateTag5.symbols
@@ -29,7 +29,8 @@ Multimap.prototype = {
 >key : Symbol(key, Decl(a.js, 16, 8))
 
         return this._map[key + ''];
->this : Symbol(__object, Decl(a.js, 11, 20))
+>this._map : Symbol(Multimap._map, Decl(a.js, 6, 21))
+>_map : Symbol(Multimap._map, Decl(a.js, 6, 21))
 >key : Symbol(key, Decl(a.js, 16, 8))
     }
 }
@@ -64,7 +65,8 @@ Multimap2.prototype = {
 >key : Symbol(key, Decl(a.js, 37, 18))
 
         return this._map[key + ''];
->this : Symbol(__object, Decl(a.js, 32, 21))
+>this._map : Symbol(Multimap2._map, Decl(a.js, 27, 28))
+>_map : Symbol(Multimap2._map, Decl(a.js, 27, 28))
 >key : Symbol(key, Decl(a.js, 37, 18))
     }
 }
@@ -106,7 +108,8 @@ Ns.Multimap3.prototype = {
 >key : Symbol(key, Decl(a.js, 59, 8))
 
         return this._map[key + ''];
->this : Symbol(__object, Decl(a.js, 54, 24))
+>this._map : Symbol(Multimap3._map, Decl(a.js, 49, 27))
+>_map : Symbol(Multimap3._map, Decl(a.js, 49, 27))
 >key : Symbol(key, Decl(a.js, 59, 8))
     }
 }
diff --git a/tests/baselines/reference/jsdocTemplateTag5.types b/tests/baselines/reference/jsdocTemplateTag5.types
index c5b8752d2b62f..1195b372e18c3 100644
--- a/tests/baselines/reference/jsdocTemplateTag5.types
+++ b/tests/baselines/reference/jsdocTemplateTag5.types
@@ -34,10 +34,10 @@ Multimap.prototype = {
 >key : K
 
         return this._map[key + ''];
->this._map[key + ''] : any
->this._map : any
->this : { get(key: K): V; }
->_map : any
+>this._map[key + ''] : V
+>this._map : { [x: string]: V; }
+>this : Multimap & { get(key: K): V; }
+>_map : { [x: string]: V; }
 >key + '' : string
 >key : K
 >'' : ""
@@ -81,10 +81,10 @@ Multimap2.prototype = {
 >key : K
 
         return this._map[key + ''];
->this._map[key + ''] : any
->this._map : any
->this : { get: (key: K) => V; }
->_map : any
+>this._map[key + ''] : V
+>this._map : { [x: string]: V; }
+>this : Multimap2 & { get: (key: K) => V; }
+>_map : { [x: string]: V; }
 >key + '' : string
 >key : K
 >'' : ""
@@ -136,10 +136,10 @@ Ns.Multimap3.prototype = {
 >key : K
 
         return this._map[key + ''];
->this._map[key + ''] : any
->this._map : any
->this : { get(key: K): V; }
->_map : any
+>this._map[key + ''] : V
+>this._map : { [x: string]: V; }
+>this : Multimap3 & { get(key: K): V; }
+>_map : { [x: string]: V; }
 >key + '' : string
 >key : K
 >'' : ""
diff --git a/tests/baselines/reference/typeFromPrototypeAssignment.errors.txt b/tests/baselines/reference/typeFromPrototypeAssignment.errors.txt
new file mode 100644
index 0000000000000..edd3435c445c3
--- /dev/null
+++ b/tests/baselines/reference/typeFromPrototypeAssignment.errors.txt
@@ -0,0 +1,45 @@
+tests/cases/conformance/salsa/a.js(27,20): error TS2339: Property 'addon' does not exist on type '{ set: () => void; get(): void; }'.
+
+
+==== tests/cases/conformance/salsa/a.js (1 errors) ====
+    // all references to _map, set, get, addon should be ok
+    
+    /** @constructor */
+    var Multimap = function() {
+        this._map = {};
+        this._map
+        this.set
+        this.get
+        this.addon
+    };
+    
+    Multimap.prototype = {
+        set: function() {
+            this._map
+            this.set
+            this.get
+            this.addon
+        },
+        get() {
+            this._map
+            this.set
+            this.get
+            this.addon
+        }
+    }
+    
+    Multimap.prototype.addon = function () {
+                       ~~~~~
+!!! error TS2339: Property 'addon' does not exist on type '{ set: () => void; get(): void; }'.
+        this._map
+        this.set
+        this.get
+        this.addon
+    }
+    
+    var mm = new Multimap();
+    mm._map
+    mm.set
+    mm.get
+    mm.addon
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/typeFromPrototypeAssignment.symbols b/tests/baselines/reference/typeFromPrototypeAssignment.symbols
new file mode 100644
index 0000000000000..d51382a639fca
--- /dev/null
+++ b/tests/baselines/reference/typeFromPrototypeAssignment.symbols
@@ -0,0 +1,122 @@
+=== tests/cases/conformance/salsa/a.js ===
+// all references to _map, set, get, addon should be ok
+
+/** @constructor */
+var Multimap = function() {
+>Multimap : Symbol(Multimap, Decl(a.js, 3, 3), Decl(a.js, 9, 2))
+
+    this._map = {};
+>this._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+    this._map
+>this._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+    this.set
+>this.set : Symbol(set, Decl(a.js, 11, 22))
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+    this.get
+>this.get : Symbol(get, Decl(a.js, 17, 6))
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+    this.addon
+>this.addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+
+};
+
+Multimap.prototype = {
+>Multimap.prototype : Symbol(Multimap.prototype, Decl(a.js, 9, 2))
+>Multimap : Symbol(Multimap, Decl(a.js, 3, 3), Decl(a.js, 9, 2))
+>prototype : Symbol(Multimap.prototype, Decl(a.js, 9, 2))
+
+    set: function() {
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+        this._map
+>this._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+        this.set
+>this.set : Symbol(set, Decl(a.js, 11, 22))
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+        this.get
+>this.get : Symbol(get, Decl(a.js, 17, 6))
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+        this.addon
+>this.addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+
+    },
+    get() {
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+        this._map
+>this._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+        this.set
+>this.set : Symbol(set, Decl(a.js, 11, 22))
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+        this.get
+>this.get : Symbol(get, Decl(a.js, 17, 6))
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+        this.addon
+>this.addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+    }
+}
+
+Multimap.prototype.addon = function () {
+>Multimap.prototype : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>Multimap : Symbol(Multimap, Decl(a.js, 3, 3), Decl(a.js, 9, 2))
+>prototype : Symbol(Multimap.prototype, Decl(a.js, 9, 2))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+
+    this._map
+>this._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+    this.set
+>this.set : Symbol(set, Decl(a.js, 11, 22))
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+    this.get
+>this.get : Symbol(get, Decl(a.js, 17, 6))
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+    this.addon
+>this.addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+}
+
+var mm = new Multimap();
+>mm : Symbol(mm, Decl(a.js, 33, 3))
+>Multimap : Symbol(Multimap, Decl(a.js, 3, 3), Decl(a.js, 9, 2))
+
+mm._map
+>mm._map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+>mm : Symbol(mm, Decl(a.js, 33, 3))
+>_map : Symbol(Multimap._map, Decl(a.js, 3, 27))
+
+mm.set
+>mm.set : Symbol(set, Decl(a.js, 11, 22))
+>mm : Symbol(mm, Decl(a.js, 33, 3))
+>set : Symbol(set, Decl(a.js, 11, 22))
+
+mm.get
+>mm.get : Symbol(get, Decl(a.js, 17, 6))
+>mm : Symbol(mm, Decl(a.js, 33, 3))
+>get : Symbol(get, Decl(a.js, 17, 6))
+
+mm.addon
+>mm.addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+>mm : Symbol(mm, Decl(a.js, 33, 3))
+>addon : Symbol(Multimap.addon, Decl(a.js, 24, 1))
+
diff --git a/tests/baselines/reference/typeFromPrototypeAssignment.types b/tests/baselines/reference/typeFromPrototypeAssignment.types
new file mode 100644
index 0000000000000..87e4be5e0b96f
--- /dev/null
+++ b/tests/baselines/reference/typeFromPrototypeAssignment.types
@@ -0,0 +1,149 @@
+=== tests/cases/conformance/salsa/a.js ===
+// all references to _map, set, get, addon should be ok
+
+/** @constructor */
+var Multimap = function() {
+>Multimap : typeof Multimap
+>function() {    this._map = {};    this._map    this.set    this.get    this.addon} : typeof Multimap
+
+    this._map = {};
+>this._map = {} : {}
+>this._map : {}
+>this : Multimap & { set: () => void; get(): void; }
+>_map : {}
+>{} : {}
+
+    this._map
+>this._map : {}
+>this : Multimap & { set: () => void; get(): void; }
+>_map : {}
+
+    this.set
+>this.set : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>set : () => void
+
+    this.get
+>this.get : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>get : () => void
+
+    this.addon
+>this.addon : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>addon : () => void
+
+};
+
+Multimap.prototype = {
+>Multimap.prototype = {    set: function() {        this._map        this.set        this.get        this.addon    },    get() {        this._map        this.set        this.get        this.addon    }} : { set: () => void; get(): void; }
+>Multimap.prototype : { set: () => void; get(): void; }
+>Multimap : typeof Multimap
+>prototype : { set: () => void; get(): void; }
+>{    set: function() {        this._map        this.set        this.get        this.addon    },    get() {        this._map        this.set        this.get        this.addon    }} : { set: () => void; get(): void; }
+
+    set: function() {
+>set : () => void
+>function() {        this._map        this.set        this.get        this.addon    } : () => void
+
+        this._map
+>this._map : {}
+>this : Multimap & { set: () => void; get(): void; }
+>_map : {}
+
+        this.set
+>this.set : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>set : () => void
+
+        this.get
+>this.get : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>get : () => void
+
+        this.addon
+>this.addon : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>addon : () => void
+
+    },
+    get() {
+>get : () => void
+
+        this._map
+>this._map : {}
+>this : Multimap & { set: () => void; get(): void; }
+>_map : {}
+
+        this.set
+>this.set : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>set : () => void
+
+        this.get
+>this.get : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>get : () => void
+
+        this.addon
+>this.addon : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>addon : () => void
+    }
+}
+
+Multimap.prototype.addon = function () {
+>Multimap.prototype.addon = function () {    this._map    this.set    this.get    this.addon} : () => void
+>Multimap.prototype.addon : any
+>Multimap.prototype : { set: () => void; get(): void; }
+>Multimap : typeof Multimap
+>prototype : { set: () => void; get(): void; }
+>addon : any
+>function () {    this._map    this.set    this.get    this.addon} : () => void
+
+    this._map
+>this._map : {}
+>this : Multimap & { set: () => void; get(): void; }
+>_map : {}
+
+    this.set
+>this.set : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>set : () => void
+
+    this.get
+>this.get : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>get : () => void
+
+    this.addon
+>this.addon : () => void
+>this : Multimap & { set: () => void; get(): void; }
+>addon : () => void
+}
+
+var mm = new Multimap();
+>mm : Multimap & { set: () => void; get(): void; }
+>new Multimap() : Multimap & { set: () => void; get(): void; }
+>Multimap : typeof Multimap
+
+mm._map
+>mm._map : {}
+>mm : Multimap & { set: () => void; get(): void; }
+>_map : {}
+
+mm.set
+>mm.set : () => void
+>mm : Multimap & { set: () => void; get(): void; }
+>set : () => void
+
+mm.get
+>mm.get : () => void
+>mm : Multimap & { set: () => void; get(): void; }
+>get : () => void
+
+mm.addon
+>mm.addon : () => void
+>mm : Multimap & { set: () => void; get(): void; }
+>addon : () => void
+
diff --git a/tests/cases/conformance/salsa/typeFromPrototypeAssignment.ts b/tests/cases/conformance/salsa/typeFromPrototypeAssignment.ts
new file mode 100644
index 0000000000000..373ccab0394bf
--- /dev/null
+++ b/tests/cases/conformance/salsa/typeFromPrototypeAssignment.ts
@@ -0,0 +1,44 @@
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+// @Filename: a.js
+// @strict: true
+
+// all references to _map, set, get, addon should be ok
+
+/** @constructor */
+var Multimap = function() {
+    this._map = {};
+    this._map
+    this.set
+    this.get
+    this.addon
+};
+
+Multimap.prototype = {
+    set: function() {
+        this._map
+        this.set
+        this.get
+        this.addon
+    },
+    get() {
+        this._map
+        this.set
+        this.get
+        this.addon
+    }
+}
+
+Multimap.prototype.addon = function () {
+    this._map
+    this.set
+    this.get
+    this.addon
+}
+
+var mm = new Multimap();
+mm._map
+mm.set
+mm.get
+mm.addon