diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 84321b4b624..ffa010c6551 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -994,11 +994,6 @@ namespace ts {
 
     /**
      * Gets the owned, enumerable property keys of a map-like.
-     *
-     * NOTE: This is intended for use with MapLike<T> objects. For Map<T> objects, use
-     *       Object.keys instead as it offers better performance.
-     *
-     * @param map A map-like.
      */
     export function getOwnKeys<T>(map: MapLike<T>): string[] {
         const keys: string[] = [];
@@ -1011,6 +1006,17 @@ namespace ts {
         return keys;
     }
 
+    export function getOwnValues<T>(sparseArray: T[]): T[] {
+        const values: T[] = [];
+        for (const key in sparseArray) {
+            if (hasOwnProperty.call(sparseArray, key)) {
+                values.push(sparseArray[key]);
+            }
+        }
+
+        return values;
+    }
+
     /** Shims `Array.from`. */
     export function arrayFrom<T, U>(iterator: Iterator<T>, map: (t: T) => U): U[];
     export function arrayFrom<T>(iterator: Iterator<T>): T[];
diff --git a/src/compiler/symbolWalker.ts b/src/compiler/symbolWalker.ts
index e20ef9f9d98..ac6b60bdc3e 100644
--- a/src/compiler/symbolWalker.ts
+++ b/src/compiler/symbolWalker.ts
@@ -14,21 +14,29 @@ namespace ts {
         return getSymbolWalker;
 
         function getSymbolWalker(accept: (symbol: Symbol) => boolean = () => true): SymbolWalker {
-            const visitedTypes = createMap<Type>(); // Key is id as string
-            const visitedSymbols = createMap<Symbol>(); // Key is id as string
+            const visitedTypes: Type[] = []; // Sparse array from id to type
+            const visitedSymbols: Symbol[] = []; // Sparse array from id to symbol
 
             return {
                 walkType: type => {
-                    visitedTypes.clear();
-                    visitedSymbols.clear();
-                    visitType(type);
-                    return { visitedTypes: arrayFrom(visitedTypes.values()), visitedSymbols: arrayFrom(visitedSymbols.values()) };
+                    try {
+                        visitType(type);
+                        return { visitedTypes: getOwnValues(visitedTypes), visitedSymbols: getOwnValues(visitedSymbols) };
+                    }
+                    finally {
+                        clear(visitedTypes);
+                        clear(visitedSymbols);
+                    }
                 },
                 walkSymbol: symbol => {
-                    visitedTypes.clear();
-                    visitedSymbols.clear();
-                    visitSymbol(symbol);
-                    return { visitedTypes: arrayFrom(visitedTypes.values()), visitedSymbols: arrayFrom(visitedSymbols.values()) };
+                    try {
+                        visitSymbol(symbol);
+                        return { visitedTypes: getOwnValues(visitedTypes), visitedSymbols: getOwnValues(visitedSymbols) };
+                    }
+                    finally {
+                        clear(visitedTypes);
+                        clear(visitedSymbols);
+                    }
                 },
             };
 
@@ -37,11 +45,10 @@ namespace ts {
                     return;
                 }
 
-                const typeIdString = type.id.toString();
-                if (visitedTypes.has(typeIdString)) {
+                if (visitedTypes[type.id]) {
                     return;
                 }
-                visitedTypes.set(typeIdString, type);
+                visitedTypes[type.id] = type;
 
                 // Reuse visitSymbol to visit the type's symbol,
                 //  but be sure to bail on recuring into the type if accept declines the symbol.
@@ -79,18 +86,9 @@ namespace ts {
                 }
             }
 
-            function visitTypeList(types: Type[]): void {
-                if (!types) {
-                    return;
-                }
-                for (let i = 0; i < types.length; i++) {
-                    visitType(types[i]);
-                }
-            }
-
             function visitTypeReference(type: TypeReference): void {
                 visitType(type.target);
-                visitTypeList(type.typeArguments);
+                forEach(type.typeArguments, visitType);
             }
 
             function visitTypeParameter(type: TypeParameter): void {
@@ -98,7 +96,7 @@ namespace ts {
             }
 
             function visitUnionOrIntersectionType(type: UnionOrIntersectionType): void {
-                visitTypeList(type.types);
+                forEach(type.types, visitType);
             }
 
             function visitIndexType(type: IndexType): void {
@@ -122,7 +120,7 @@ namespace ts {
                 if (signature.typePredicate) {
                     visitType(signature.typePredicate.type);
                 }
-                visitTypeList(signature.typeParameters);
+                forEach(signature.typeParameters, visitType);
 
                 for (const parameter of signature.parameters){
                     visitSymbol(parameter);
@@ -133,8 +131,8 @@ namespace ts {
 
             function visitInterfaceType(interfaceT: InterfaceType): void {
                 visitObjectType(interfaceT);
-                visitTypeList(interfaceT.typeParameters);
-                visitTypeList(getBaseTypes(interfaceT));
+                forEach(interfaceT.typeParameters, visitType);
+                forEach(getBaseTypes(interfaceT), visitType);
                 visitType(interfaceT.thisType);
             }
 
@@ -161,11 +159,11 @@ namespace ts {
                 if (!symbol) {
                     return;
                 }
-                const symbolIdString = getSymbolId(symbol).toString();
-                if (visitedSymbols.has(symbolIdString)) {
+                const symbolId = getSymbolId(symbol);
+                if (visitedSymbols[symbolId]) {
                     return;
                 }
-                visitedSymbols.set(symbolIdString, symbol);
+                visitedSymbols[symbolId] = symbol;
                 if (!accept(symbol)) {
                     return true;
                 }