diff --git a/package.json b/package.json
index e0d7391..3efa1ec 100644
--- a/package.json
+++ b/package.json
@@ -75,7 +75,7 @@
     "ts-jest": "^24.3.0",
     "tslint": "^5.20.1",
     "typedoc": "^0.16.2",
-    "typescript": "^3.9.2"
+    "typescript": "^4.0.5"
   },
   "babel": {
     "presets": [
diff --git a/src/store/BasicStore.ts b/src/store/BasicStore.ts
index 4e7fc26..341f90a 100644
--- a/src/store/BasicStore.ts
+++ b/src/store/BasicStore.ts
@@ -19,6 +19,9 @@ export default class BasicStore implements LowLevelStore {
     public readonly dataCallbacks: Array<(quad: Quad) => void>;
     public readonly removeCallback: ((quad: Quad) => void) | undefined;
 
+    /** Returns the number of quads in the store. */
+    public length: number = 0;
+
     constructor(opts: Partial<IndexedFormulaOpts> = {}) {
         this.dataCallbacks = [];
         this.quads = opts.quads || [];
@@ -26,6 +29,15 @@ export default class BasicStore implements LowLevelStore {
         this.rdfArrayRemove = this.rdfFactory.supports?.[Feature.identity]
             ? this.identityRemove
             : this.searchRemove;
+
+        Object.defineProperty(this, "length", {
+            get(): number {
+                return this.quads.length;
+            },
+            set(value: number): void {
+                this.quads.length = value;
+            },
+        });
     }
 
     /** Add a quad to the store. */
@@ -78,11 +90,6 @@ export default class BasicStore implements LowLevelStore {
         this.dataCallbacks.push(callback);
     }
 
-    /** Returns the number of quads in the store. */
-    public get length(): number {
-        return this.quads.length;
-    }
-
     /** Remove a quad from the store */
     public remove(st: Quad): this {
         const sts = this.match(
diff --git a/src/store/RDFIndex.ts b/src/store/RDFIndex.ts
index 20512be..d5673ee 100644
--- a/src/store/RDFIndex.ts
+++ b/src/store/RDFIndex.ts
@@ -20,6 +20,9 @@ export type PropertyActionCallback = (quad: Quad) => void;
 
 /** Query and modify an array of quads. */
 export default class RDFIndex extends Equatable(Indexable(BasicStore)) implements LowLevelStore {
+    /** Returns the number of quads in the store. */
+    public length: number = 0;
+
     private readonly propertyActions: PropertyActionCallback[][] = [];
 
     /**
@@ -29,7 +32,17 @@ export default class RDFIndex extends Equatable(Indexable(BasicStore)) implement
      */
     constructor(opts: Partial<IndexedFormulaOpts> = {}) {
         super(opts);
+
         this.addDataCallback(this.processPropertyAction.bind(this));
+
+        Object.defineProperty(this, "length", {
+            get(): number {
+                return this.quads.length;
+            },
+            set(value: number): void {
+                this.quads.length = value;
+            },
+        });
     }
 
     public any(
@@ -64,20 +77,6 @@ export default class RDFIndex extends Equatable(Indexable(BasicStore)) implement
         return this.match(subject, predicate, object, graph, true)?.[0];
     }
 
-    /**
-     * Returns the number of quads contained in this IndexedFormula.
-     * (Getter proxy to this.quads).
-     * Usage:
-     *    ```
-     *    const kb = rdf.graph()
-     *    kb.length  // -> 0
-     *    ```
-     * @returns {Number}
-     */
-    public get length(): number {
-        return this.quads.length;
-    }
-
     public holds(s: SomeNode, p: NamedNode, o: SomeTerm, g: SomeNode): boolean {
         return this.match(s, p, o, g, true)?.[0] !== undefined;
     }
diff --git a/yarn.lock b/yarn.lock
index 4818ad9..392944e 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -6550,10 +6550,10 @@ typescript@3.7.x:
   resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.7.5.tgz#0692e21f65fd4108b9330238aac11dd2e177a1ae"
   integrity sha512-/P5lkRXkWHNAbcJIiHPfRoKqyd7bsyCma1hZNUGfn20qm64T6ZBlrzprymeu918H+mB/0rIg2gGK/BXkhhYgBw==
 
-typescript@^3.9.2:
-  version "3.9.2"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.9.2.tgz#64e9c8e9be6ea583c54607677dd4680a1cf35db9"
-  integrity sha512-q2ktq4n/uLuNNShyayit+DTobV2ApPEo/6so68JaD5ojvc/6GClBipedB9zNWYxRSAlZXAe405Rlijzl6qDiSw==
+typescript@^4.0.5:
+  version "4.0.5"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-4.0.5.tgz#ae9dddfd1069f1cb5beb3ef3b2170dd7c1332389"
+  integrity sha512-ywmr/VrTVCmNTJ6iV2LwIrfG1P+lv6luD8sUJs+2eI9NLGigaN+nUQc13iHqisq7bra9lnmUSYqbJvegraBOPQ==
 
 uglify-js@^3.1.4:
   version "3.8.0"