diff --git a/js/src/data.ts b/js/src/data.ts
index cc689ffc15d46..e05b5aefff978 100644
--- a/js/src/data.ts
+++ b/js/src/data.ts
@@ -86,7 +86,7 @@ export class BaseData<T extends DataType = DataType> implements VectorLike {
         let nullCount = this._nullCount;
         let nullBitmap: Uint8Array | undefined;
         if (nullCount === -1 && (nullBitmap = this[VectorType.VALIDITY])) {
-            this._nullCount = nullCount = popcnt_bit_range(nullBitmap, this._offset, this._offset + this._length);
+            this._nullCount = nullCount = this._length - popcnt_bit_range(nullBitmap, this._offset, this._offset + this._length);
         }
         return nullCount;
     }
diff --git a/js/src/type.ts b/js/src/type.ts
index 4b317ed7fab5e..6f382bd5b2b05 100644
--- a/js/src/type.ts
+++ b/js/src/type.ts
@@ -201,7 +201,9 @@ export abstract class DataType<TType extends Type = any> implements Partial<Visi
 
 export interface Null extends DataType<Type.Null> { TArray: void; TValue: null; }
 export class Null extends DataType<Type.Null> {
-    constructor() { super(Type.Null); }
+    constructor() {
+        super(Type.Null);
+    }
     public toString() { return `Null`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any {
         return visitor.visitNull(this);
@@ -269,7 +271,9 @@ export class Float64 extends Float<Float64Array> { constructor() { super(Precisi
 
 export interface Binary extends DataType<Type.Binary> { TArray: Uint8Array; TValue: Uint8Array; }
 export class Binary extends DataType<Type.Binary> {
-    constructor() { super(Type.Binary); }
+    constructor() {
+        super(Type.Binary);
+    }
     public toString() { return `Binary`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any {
         return visitor.visitBinary(this);
@@ -282,7 +286,9 @@ export class Binary extends DataType<Type.Binary> {
 
 export interface Utf8 extends DataType<Type.Utf8> { TArray: Uint8Array; TValue: string; }
 export class Utf8 extends DataType<Type.Utf8> {
-    constructor() { super(Type.Utf8); }
+    constructor() {
+        super(Type.Utf8);
+    }
     public toString() { return `Utf8`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any {
         return visitor.visitUtf8(this);
@@ -295,7 +301,9 @@ export class Utf8 extends DataType<Type.Utf8> {
 
 export interface Bool extends DataType<Type.Bool> { TArray: Uint8Array; TValue: boolean; }
 export class Bool extends DataType<Type.Bool> {
-    constructor() { super(Type.Bool); }
+    constructor() {
+        super(Type.Bool);
+    }
     public toString() { return `Bool`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any {
         return visitor.visitBool(this);
@@ -325,7 +333,9 @@ export class Decimal extends DataType<Type.Decimal> {
 /* tslint:disable:class-name */
 export interface Date_ extends DataType<Type.Date> { TArray: Int32Array; TValue: Date; }
 export class Date_ extends DataType<Type.Date> {
-    constructor(public readonly unit: DateUnit) { super(Type.Date); }
+    constructor(public readonly unit: DateUnit) {
+        super(Type.Date);
+    }
     public toString() { return `Date${(this.unit + 1) * 32}<${DateUnit[this.unit]}>`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any {
         return visitor.visitDate(this);
@@ -485,7 +495,7 @@ export class Map_ extends DataType<Type.Map> {
     public toString() { return `Map<${this.children.join(`, `)}>`; }
     public acceptTypeVisitor(visitor: TypeVisitor): any { return visitor.visitMap(this); }
     protected static [Symbol.toStringTag] = ((proto: Map_) => {
-        return proto[Symbol.toStringTag] = 'Map';
+        return proto[Symbol.toStringTag] = 'Map_';
     })(Map_.prototype);
 }
 
diff --git a/js/src/vector/flat.ts b/js/src/vector/flat.ts
index 800dbd5479145..a32bd9d397ff0 100644
--- a/js/src/vector/flat.ts
+++ b/js/src/vector/flat.ts
@@ -41,10 +41,10 @@ export class FlatView<T extends FlatType> implements View<T> {
         return this.values[index] = value;
     }
     public toArray(): IterableArrayLike<T['TValue']> {
-        return this.values;
+        return this.values.subarray(0, this.length);
     }
     public [Symbol.iterator](): IterableIterator<T['TValue']> {
-        return this.values[Symbol.iterator]() as IterableIterator<T['TValue']>;
+        return this.values.subarray(0, this.length)[Symbol.iterator]() as IterableIterator<T['TValue']>;
     }
 }
 
@@ -152,7 +152,9 @@ export class PrimitiveView<T extends PrimitiveType> extends FlatView<T> {
         return this.setValue(this.values, index, this.size, value);
     }
     public toArray(): IterableArrayLike<T['TValue']> {
-        return this.size === 1 ? this.values : new this.ArrayType(this);
+        return this.size > 1 ?
+            new this.ArrayType(this) :
+            this.values.subarray(0, this.length);
     }
     public *[Symbol.iterator](): IterableIterator<T['TValue']> {
         const get = this.getValue;
diff --git a/js/src/vector/list.ts b/js/src/vector/list.ts
index 0ea5a93befb73..3d365ceac29b2 100644
--- a/js/src/vector/list.ts
+++ b/js/src/vector/list.ts
@@ -65,7 +65,7 @@ export abstract class ListViewBase<T extends (ListType | FlatListType | FixedSiz
 
 export abstract class VariableListViewBase<T extends (ListType | FlatListType)> extends ListViewBase<T> {
     constructor(data: Data<T>) {
-        super(data)
+        super(data);
         this.length = data.length;
         this.valueOffsets = data.valueOffsets;
     }