diff --git a/js/src/builder/union.ts b/js/src/builder/union.ts
index 7bee460a77d..6b6f4d476dc 100644
--- a/js/src/builder/union.ts
+++ b/js/src/builder/union.ts
@@ -56,6 +56,7 @@ export abstract class UnionBuilder<T extends Union, TNull = any> extends Builder
         const childIndex = this.type.typeIdToChildIndex[childTypeId!];
         const child = this.children[childIndex];
         child?.set(index, value);
+        this.length = Math.max(index + 1, this.length);
     }
 
     public addChild(child: Builder, name = `${this.children.length}`) {
@@ -93,5 +94,6 @@ export class DenseUnionBuilder<T extends DenseUnion, TNull = any> extends UnionB
         const child = this.getChildAt(this.type.typeIdToChildIndex[id])!;
         const denseIndex = this._offsets.set(index, child.length).buffer[index];
         child?.set(denseIndex, value);
+        this.length = Math.max(index + 1, this.length);
     }
 }
diff --git a/js/test/unit/builders/utils.ts b/js/test/unit/builders/utils.ts
index 7cc0a075d84..1ea6a4e1f26 100644
--- a/js/test/unit/builders/utils.ts
+++ b/js/test/unit/builders/utils.ts
@@ -141,6 +141,7 @@ export function validateVector<T extends DataType>(vals: (T['TValue'] | null)[],
     let i = 0, x: T['TValue'] | null, y: T['TValue'] | null;
     const nulls = nullVals.reduce((m, x) => m.set(x, x), new Map());
     try {
+        expect(vec).toHaveLength(vals.length);
         for (x of vec) {
             if (nulls.has(y = vals[i])) {
                 expect(x).toBeNull();