diff --git a/runtime/JavaScript/spec/HashMapSpec.js b/runtime/JavaScript/spec/HashMapSpec.js
new file mode 100644
index 0000000000..0f328ad9d9
--- /dev/null
+++ b/runtime/JavaScript/spec/HashMapSpec.js
@@ -0,0 +1,60 @@
+import HashMap from "../src/antlr4/misc/HashMap.js";
+import HashCode from "../src/antlr4/misc/HashCode.js";
+
+class Thing {
+
+    value1 = Math.random();
+    value2 = Math.random();
+
+    hashCode() {
+        return HashCode.hashStuff(this.value1);
+    }
+
+    equals(other) {
+        return other instanceof Thing
+            && other.value1 === this.value1
+            && other.value2 === this.value2;
+    }
+}
+
+describe('test HashMap', () => {
+
+    it("sets a thing", () => {
+        const t1 = new Thing();
+        const map = new HashMap();
+        map.set("abc", t1);
+        expect(map.containsKey("abc")).toBeTrue();
+        expect(map.containsKey("def")).toBeFalse();
+        expect(map.length).toEqual(1);
+    })
+
+    it("gets a thing", () => {
+        const t1 = new Thing();
+        const map = new HashMap();
+        map.set("abc", t1);
+        const t2 = map.get("abc");
+        expect(t2).toEqual(t1);
+    })
+
+    it("replaces a thing", () => {
+        const t1 = new Thing();
+        const t2 = new Thing();
+        const map = new HashMap();
+        map.set("abc", t1);
+        map.set("abc", t2);
+        const t3 = map.get("abc");
+        expect(t3).toEqual(t2);
+    })
+
+    it("returns correct length", () => {
+        const t1 = new Thing();
+        const t2 = new Thing();
+        const map = new HashMap();
+        expect(map.length).toEqual(0);
+        map.set("abc", t1);
+        expect(map.length).toEqual(1);
+        map.set("def", t2);
+        expect(map.length).toEqual(2);
+    })
+
+});
diff --git a/runtime/JavaScript/src/antlr4/misc/HashMap.js b/runtime/JavaScript/src/antlr4/misc/HashMap.js
index 13c46cada2..8740816a2e 100644
--- a/runtime/JavaScript/src/antlr4/misc/HashMap.js
+++ b/runtime/JavaScript/src/antlr4/misc/HashMap.js
@@ -5,80 +5,110 @@
 import standardEqualsFunction from "../utils/standardEqualsFunction.js";
 import standardHashCodeFunction from "../utils/standardHashCodeFunction.js";
 
-const HASH_KEY_PREFIX = "h-";
+const DEFAULT_LOAD_FACTOR = 0.75;
+const INITIAL_CAPACITY = 16
 
 export default class HashMap {
 
     constructor(hashFunction, equalsFunction) {
-        this.data = {};
+        this.buckets = new Array(INITIAL_CAPACITY);
+        this.threshold = Math.floor(INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
+        this.itemCount = 0;
         this.hashFunction = hashFunction || standardHashCodeFunction;
         this.equalsFunction = equalsFunction || standardEqualsFunction;
     }
 
     set(key, value) {
-        const hashKey = HASH_KEY_PREFIX + this.hashFunction(key);
-        if (hashKey in this.data) {
-            const entries = this.data[hashKey];
-            for (let i = 0; i < entries.length; i++) {
-                const entry = entries[i];
-                if (this.equalsFunction(key, entry.key)) {
-                    const oldValue = entry.value;
-                    entry.value = value;
-                    return oldValue;
-                }
-            }
-            entries.push({key:key, value:value});
+        this._expand();
+        const slot = this._getSlot(key);
+        let bucket = this.buckets[slot];
+        if (!bucket) {
+            bucket = [[key, value]];
+            this.buckets[slot] = bucket;
+            this.itemCount++;
             return value;
+        }
+        const existing = bucket.find(pair => this.equalsFunction(pair[0], key), this);
+        if(existing) {
+            const result = existing[1];
+            existing[1] = value;
+            return result;
         } else {
-            this.data[hashKey] = [{key:key, value:value}];
+            bucket.push([key, value]);
+            this.itemCount++;
             return value;
         }
     }
 
     containsKey(key) {
-        const hashKey = HASH_KEY_PREFIX + this.hashFunction(key);
-        if(hashKey in this.data) {
-            const entries = this.data[hashKey];
-            for (let i = 0; i < entries.length; i++) {
-                const entry = entries[i];
-                if (this.equalsFunction(key, entry.key))
-                    return true;
-            }
+        const bucket = this._getBucket(key);
+        if(!bucket) {
+            return false;
         }
-        return false;
+        const existing = bucket.find(pair => this.equalsFunction(pair[0], key), this);
+        return !!existing;
     }
 
     get(key) {
-        const hashKey = HASH_KEY_PREFIX + this.hashFunction(key);
-        if(hashKey in this.data) {
-            const entries = this.data[hashKey];
-            for (let i = 0; i < entries.length; i++) {
-                const entry = entries[i];
-                if (this.equalsFunction(key, entry.key))
-                    return entry.value;
-            }
+        const bucket = this._getBucket(key);
+        if(!bucket) {
+            return null;
         }
-        return null;
+        const existing = bucket.find(pair => this.equalsFunction(pair[0], key), this);
+        return existing ? existing[1] : null;
     }
 
     entries() {
-        return Object.keys(this.data).filter(key => key.startsWith(HASH_KEY_PREFIX)).flatMap(key => this.data[key], this);
+        return this.buckets.filter(b => b != null).flat(1);
     }
 
     getKeys() {
-        return this.entries().map(e => e.key);
+        return this.entries().map(pair => pair[0]);
     }
 
     getValues() {
-        return this.entries().map(e => e.value);
+        return this.entries().map(pair => pair[1]);
     }
 
     toString() {
-        const ss = this.entries().map(e => '{' + e.key + ':' + e.value + '}');
+        const ss = this.entries().map(e => '{' + e[0] + ':' + e[1] + '}');
         return '[' + ss.join(", ") + ']';
     }
 
     get length() {
-        return Object.keys(this.data).filter(key => key.startsWith(HASH_KEY_PREFIX)).map(key => this.data[key].length, this).reduce((accum, item) => accum + item, 0);
+        return this.itemCount;
+    }
+
+    _getSlot(key) {
+        const hash = this.hashFunction(key);
+        return hash & this.buckets.length - 1;
+    }
+    _getBucket(key) {
+        return this.buckets[this._getSlot(key)];
     }
+
+    _expand() {
+        if (this.itemCount <= this.threshold) {
+            return;
+        }
+        const old_buckets = this.buckets;
+        const newCapacity = this.buckets.length * 2;
+        this.buckets = new Array(newCapacity);
+        this.threshold = Math.floor(newCapacity * DEFAULT_LOAD_FACTOR);
+        for (const bucket of old_buckets) {
+            if (!bucket) {
+                continue;
+            }
+            for (const pair of bucket) {
+                const slot = this._getSlot(pair[0]);
+                let newBucket = this.buckets[slot];
+                if (!newBucket) {
+                    newBucket = [];
+                    this.buckets[slot] = newBucket;
+                }
+                newBucket.push(pair);
+            }
+        }
+    }
+
 }
diff --git a/runtime/JavaScript/src/antlr4/utils/standardEqualsFunction.js b/runtime/JavaScript/src/antlr4/utils/standardEqualsFunction.js
index 3269df835e..a6efbd5a24 100644
--- a/runtime/JavaScript/src/antlr4/utils/standardEqualsFunction.js
+++ b/runtime/JavaScript/src/antlr4/utils/standardEqualsFunction.js
@@ -3,5 +3,5 @@
  * can be found in the LICENSE.txt file in the project root.
  */
 export default function standardEqualsFunction(a, b) {
-    return a ? a.equals(b) : a===b;
+    return a && a.equals ? a.equals(b) : a===b;
 }