diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index eb17740277119..092fa597e327d 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -566,6 +566,21 @@ namespace ts {
         return result;
     }
 
+    /**
+     * Adds the value to an array of values associated with the key, and returns the array.
+     * Creates the array if it does not already exist.
+     */
+    export function multiMapAdd<V>(map: Map<V[]>, key: string, value: V): V[] {
+        const values = map[key];
+        if (values) {
+            values.push(value);
+            return values;
+        }
+        else {
+            return map[key] = [value];
+        }
+    }
+
     /**
      * Tests whether a value is an array.
      */
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index f02cf2667444b..90cc06fa4be9e 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -6851,7 +6851,7 @@ const _super = (function (geti, seti) {
                                 // export { x, y }
                                 for (const specifier of (<ExportDeclaration>node).exportClause.elements) {
                                     const name = (specifier.propertyName || specifier.name).text;
-                                    (exportSpecifiers[name] || (exportSpecifiers[name] = [])).push(specifier);
+                                    multiMapAdd(exportSpecifiers, name, specifier);
                                 }
                             }
                             break;
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 118186bf52a51..f380401c3b83b 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -267,7 +267,7 @@ namespace ts {
                 }
 
                 function addFileWatcherCallback(filePath: string, callback: FileWatcherCallback): void {
-                    (fileWatcherCallbacks[filePath] || (fileWatcherCallbacks[filePath] = [])).push(callback);
+                    multiMapAdd(fileWatcherCallbacks, filePath, callback);
                 }
 
                 function addFile(fileName: string, callback: FileWatcherCallback): WatchedFile {
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index c51b7e874f772..bd35923022087 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -1649,8 +1649,7 @@ namespace FourSlash {
             const result = ts.createMap<Range[]>();
             for (const range of this.getRanges()) {
                 const text = this.rangeText(range);
-                const ranges = result[text] || (result[text] = []);
-                ranges.push(range);
+                ts.multiMapAdd(result, text, range);
             }
             return result;
         }
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index 6814305e6ca12..a5648d999704f 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -198,9 +198,8 @@ namespace ts {
 
         watchDirectory(directoryName: string, callback: DirectoryWatcherCallback, recursive: boolean): DirectoryWatcher {
             const path = this.toPath(directoryName);
-            const callbacks = this.watchedDirectories[path] || (this.watchedDirectories[path] = []);
             const cbWithRecursive = { cb: callback, recursive };
-            callbacks.push(cbWithRecursive);
+            const callbacks = multiMapAdd(this.watchedDirectories, path, cbWithRecursive);
             return {
                 referenceCount: 0,
                 directoryName,
@@ -235,8 +234,7 @@ namespace ts {
 
         watchFile(fileName: string, callback: FileWatcherCallback) {
             const path = this.toPath(fileName);
-            const callbacks = this.watchedFiles[path] || (this.watchedFiles[path] = []);
-            callbacks.push(callback);
+            const callbacks = multiMapAdd(this.watchedFiles, path, callback);
             return {
                 close: () => {
                     unorderedRemoveItem(callbacks, callback);
diff --git a/src/services/services.ts b/src/services/services.ts
index c97455c324406..f38fa8eb3a3bc 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -987,8 +987,7 @@ namespace ts {
             function addDeclaration(declaration: Declaration) {
                 const name = getDeclarationName(declaration);
                 if (name) {
-                    const declarations = getDeclarations(name);
-                    declarations.push(declaration);
+                    multiMapAdd(result, name, declaration);
                 }
             }