diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d1821d6067..f8826da961 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9598,7 +9598,7 @@ module ts {
             return diagnostics.getDiagnostics();
         }
 
-        function getGlobalDiagnostics(): Diagnostic[]{
+        function getGlobalDiagnostics(): Diagnostic[] {
             throwIfNonDiagnosticsProducing();
             return diagnostics.getGlobalDiagnostics();
         }
@@ -9624,7 +9624,7 @@ module ts {
             return false;
         }
 
-        function getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[]{
+        function getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[] {
             var symbols: SymbolTable = {};
             var memberFlags: NodeFlags = 0;
             function copySymbol(symbol: Symbol, meaning: SymbolFlags) {
@@ -10010,7 +10010,7 @@ module ts {
             return getNamedMembers(propsByName);
         }
 
-        function getRootSymbols(symbol: Symbol): Symbol[]{
+        function getRootSymbols(symbol: Symbol): Symbol[] {
             if (symbol.flags & SymbolFlags.UnionProperty) {
                 var symbols: Symbol[] = [];
                 var name = symbol.name;
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index b82ca2f07a..d527b62005 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -342,16 +342,16 @@ module ts {
         return diagnostic.file ? diagnostic.file.fileName : undefined;
     }
 
-    export function compareDiagnostics(d1: Diagnostic, d2: Diagnostic): number {
+    export function compareDiagnostics(d1: Diagnostic, d2: Diagnostic): Comparison {
         return compareValues(getDiagnosticFileName(d1), getDiagnosticFileName(d2)) ||
             compareValues(d1.start, d2.start) ||
             compareValues(d1.length, d2.length) ||
             compareValues(d1.code, d2.code) ||
             compareMessageText(d1.messageText, d2.messageText) ||
-            0;
+            Comparison.EqualTo;
     }
 
-    function compareMessageText(text1: string | DiagnosticMessageChain, text2: string | DiagnosticMessageChain): number {
+    function compareMessageText(text1: string | DiagnosticMessageChain, text2: string | DiagnosticMessageChain): Comparison {
         while (text1 && text2) {
             // We still have both chains.
             var string1 = typeof text1 === "string" ? text1 : text1.messageText;
@@ -368,14 +368,14 @@ module ts {
 
         if (!text1 && !text2) {
             // if the chains are done, then these messages are the same.
-            return 0;
+            return Comparison.EqualTo;
         }
 
         // We still have one chain remaining.  The shorter chain should come first.
-        return text1 ? 1 : -1;
+        return text1 ? Comparison.GreaterThan : Comparison.LessThan;
     }
 
-    export function sortAndDeduplicateDiagnostics(diagnostics: Diagnostic[]): Diagnostic[] {
+    export function sortAndDeduplicateDiagnostics(diagnostics: Diagnostic[]): Diagnostic[]{
         return deduplicateSortedDiagnostics(diagnostics.sort(compareDiagnostics));
     }
 
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index e933c54bc1..954c443a77 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -165,7 +165,7 @@ module ts {
             return noDiagnosticsTypeChecker || (noDiagnosticsTypeChecker = createTypeChecker(program, /*produceDiagnostics:*/ false));
         }
 
-        function getDeclarationDiagnostics(targetSourceFile: SourceFile): Diagnostic[]{
+        function getDeclarationDiagnostics(targetSourceFile: SourceFile): Diagnostic[] {
             var resolver = getDiagnosticsProducingTypeChecker().getEmitResolver(targetSourceFile);
             return ts.getDeclarationDiagnostics(getEmitHost(), resolver, targetSourceFile);
         }
@@ -206,11 +206,11 @@ module ts {
             return sortAndDeduplicateDiagnostics(allDiagnostics);
         }
 
-        function getSyntacticDiagnostics(sourceFile?: SourceFile): Diagnostic[]{
+        function getSyntacticDiagnostics(sourceFile?: SourceFile): Diagnostic[] {
             return getDiagnosticsHelper(sourceFile, getSyntacticDiagnosticsForFile);
         }
 
-        function getSemanticDiagnostics(sourceFile?: SourceFile): Diagnostic[]{
+        function getSemanticDiagnostics(sourceFile?: SourceFile): Diagnostic[] {
             return getDiagnosticsHelper(sourceFile, getSemanticDiagnosticsForFile);
         }
 
@@ -229,7 +229,7 @@ module ts {
             return bindDiagnostics.concat(checkDiagnostics).concat(programDiagnostics);
         }
 
-        function getGlobalDiagnostics(): Diagnostic[]{
+        function getGlobalDiagnostics(): Diagnostic[] {
             var typeChecker = getDiagnosticsProducingTypeChecker();
 
             var allDiagnostics: Diagnostic[] = [];
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 23da074815..fd675597cc 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1688,7 +1688,7 @@ module ts {
     // @internal
     export interface DiagnosticCollection {
         // Adds a diagnostic to this diagnostic collection.
-        add(diagnostic?: Diagnostic): void;
+        add(diagnostic: Diagnostic): void;
 
         // Gets all the diagnostics that aren't associated with a file.
         getGlobalDiagnostics(): Diagnostic[];
@@ -1697,7 +1697,7 @@ module ts {
         // Otherwise, returns all the diagnostics (global and file associated) in this colletion.
         getDiagnostics(fileName?: string): Diagnostic[];
 
-        // Gets a count of how many times this collectoin has been modified.  This value changes
+        // Gets a count of how many times this collection has been modified.  This value changes
         // each time 'add' is called (regardless of whether or not an equivalent diagnostic was
         // already in the collection).  As such, it can be used as a simple way to tell if any
         // operation caused diagnostics to be returned by storing and comparing the return value