diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 70827becffe..a312f91c8f6 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -23552,7 +23552,7 @@ namespace ts {
             }
 
             // Initialize global symbol table
-            let augmentations: LiteralExpression[][];
+            let augmentations: ReadonlyArray<StringLiteral>[];
             for (const file of host.getSourceFiles()) {
                 if (!isExternalOrCommonJsModule(file)) {
                     mergeSymbolTable(globals, file.locals);
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 3d63edddac1..f15baeefcdb 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -3,7 +3,6 @@
 /// <reference path="core.ts" />
 
 namespace ts {
-    const emptyArray: any[] = [];
     const ignoreDiagnosticCommentRegEx = /(^\s*$)|(^\s*\/\/\/?\s*(@ts-ignore)?)/;
 
     export function findConfigFile(searchPath: string, fileExists: (fileName: string) => boolean, configName = "tsconfig.json"): string {
@@ -1390,8 +1389,8 @@ namespace ts {
             const isExternalModuleFile = isExternalModule(file);
 
             // file.imports may not be undefined if there exists dynamic import
-            let imports: LiteralExpression[];
-            let moduleAugmentations: LiteralExpression[];
+            let imports: StringLiteral[];
+            let moduleAugmentations: StringLiteral[];
             let ambientModules: string[];
 
             // If we are importing helpers, we need to add a synthetic reference to resolve the
@@ -1426,23 +1425,23 @@ namespace ts {
                     case SyntaxKind.ImportEqualsDeclaration:
                     case SyntaxKind.ExportDeclaration:
                         const moduleNameExpr = getExternalModuleName(node);
-                        if (!moduleNameExpr || moduleNameExpr.kind !== SyntaxKind.StringLiteral) {
+                        if (!moduleNameExpr || !isStringLiteral(moduleNameExpr)) {
                             break;
                         }
-                        if (!(<LiteralExpression>moduleNameExpr).text) {
+                        if (!moduleNameExpr.text) {
                             break;
                         }
 
                         // TypeScript 1.0 spec (April 2014): 12.1.6
                         // An ExternalImportDeclaration in an AmbientExternalModuleDeclaration may reference other external modules
                         // only through top - level external module names. Relative external module names are not permitted.
-                        if (!inAmbientModule || !isExternalModuleNameRelative((<LiteralExpression>moduleNameExpr).text)) {
-                            (imports || (imports = [])).push(<LiteralExpression>moduleNameExpr);
+                        if (!inAmbientModule || !isExternalModuleNameRelative(moduleNameExpr.text)) {
+                            (imports || (imports = [])).push(moduleNameExpr);
                         }
                         break;
                     case SyntaxKind.ModuleDeclaration:
                         if (isAmbientModule(<ModuleDeclaration>node) && (inAmbientModule || hasModifier(node, ModifierFlags.Ambient) || file.isDeclarationFile)) {
-                            const moduleName = <LiteralExpression>(<ModuleDeclaration>node).name;
+                            const moduleName = <StringLiteral>(<ModuleDeclaration>node).name;
                             // Ambient module declarations can be interpreted as augmentations for some existing external modules.
                             // This will happen in two cases:
                             // - if current file is external module then module augmentation is a ambient module declaration defined in the top level scope
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index e16ee34b01a..0d8041220b0 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2321,10 +2321,10 @@ namespace ts {
         // Content of this field should never be used directly - use getResolvedModuleFileName/setResolvedModuleFileName functions instead
         /* @internal */ resolvedModules: Map<ResolvedModuleFull>;
         /* @internal */ resolvedTypeReferenceDirectiveNames: Map<ResolvedTypeReferenceDirective>;
-        /* @internal */ imports: StringLiteral[];
-        /* @internal */ moduleAugmentations: StringLiteral[];
+        /* @internal */ imports: ReadonlyArray<StringLiteral>;
+        /* @internal */ moduleAugmentations: ReadonlyArray<StringLiteral>;
         /* @internal */ patternAmbientModules?: PatternAmbientModule[];
-        /* @internal */ ambientModuleNames: string[];
+        /* @internal */ ambientModuleNames: ReadonlyArray<string>;
         /* @internal */ checkJsDirective: CheckJsDirective | undefined;
     }
 
diff --git a/src/services/signatureHelp.ts b/src/services/signatureHelp.ts
index 00ab0165805..71e6bc00b00 100644
--- a/src/services/signatureHelp.ts
+++ b/src/services/signatureHelp.ts
@@ -1,8 +1,6 @@
 ///<reference path='services.ts' />
 /* @internal */
 namespace ts.SignatureHelp {
-    const emptyArray: any[] = [];
-
     export const enum ArgumentListKind {
         TypeArguments,
         CallArguments,