diff --git a/src/doov.ts b/src/doov.ts
index 4efa911..5a90f07 100644
--- a/src/doov.ts
+++ b/src/doov.ts
@@ -131,7 +131,7 @@ export function matchAny(...values: BooleanFunction[]): BooleanFunction {
   return new BooleanFunction(new NaryMetadata(values.map(value => value.metadata), MATCH_ANY), (obj, ctx) => {
     return values.some(value => {
       const v = value.get(obj, ctx);
-      return v != null ? v : false;
+      return Boolean(v);
     });
   });
 }
@@ -140,7 +140,7 @@ export function matchAll(...values: BooleanFunction[]): BooleanFunction {
   return new BooleanFunction(new NaryMetadata(values.map(value => value.metadata), MATCH_ALL), (obj, ctx) => {
     return values.every(value => {
       const v = value.get(obj, ctx);
-      return v != null ? v : false;
+      return Boolean(v);
     });
   });
 }
@@ -149,7 +149,7 @@ export function matchNone(...values: BooleanFunction[]): BooleanFunction {
   return new BooleanFunction(new NaryMetadata(values.map(value => value.metadata), NONE_MATCH), (obj, ctx) => {
     return values.every(value => {
       const v = value.get(obj, ctx);
-      return v != null ? !v : false;
+      return !Boolean(v);
     });
   });
 }
@@ -158,7 +158,7 @@ export function count(...values: BooleanFunction[]): NumberFunction {
   return new NumberFunction(new NaryMetadata(values.map(value => value.metadata), COUNT), (obj, ctx) => {
     return values.filter(value => {
       const v = value.get(obj, ctx);
-      return v != null ? v : false;
+      return Boolean(v);
     }).length;
   });
 }
@@ -167,7 +167,7 @@ export function sum(...values: NumberFunction[]): NumberFunction {
   return new NumberFunction(new NaryMetadata(values.map(value => value.metadata), SUM), (obj, ctx) => {
     return values.reduce((previous, value) => {
       const v = value.get(obj, ctx);
-      return v != null ? previous + v : previous;
+      return v !== undefined && v !== null && !isNaN(v) ? previous + v : previous;
     }, 0);
   });
 }
diff --git a/src/dsl/lang/BooleanFunction.ts b/src/dsl/lang/BooleanFunction.ts
index 06a9d4f..2b4b899 100644
--- a/src/dsl/lang/BooleanFunction.ts
+++ b/src/dsl/lang/BooleanFunction.ts
@@ -2,7 +2,7 @@ import { condition, Function } from './Function';
 import { Context } from '../Context';
 import { ContextAccessor } from '../ContextAccessor';
 import { UnaryMetadata } from '../meta/UnaryMetadata';
-import { AND, NOT, OR } from './DefaultOperators';
+import { AND, IS_FALSY, IS_TRUTHY, NOT, OR } from './DefaultOperators';
 import { BinaryMetadata } from '../meta/BinaryMetadata';
 import { ValueMetadata } from '../meta/ValueMetadata';
 
@@ -11,6 +11,20 @@ export class BooleanFunction extends Function<boolean> {
     return new BooleanFunction(accessor.metadata, accessor.get, accessor.set);
   }
 
+  public isFalsy(): BooleanFunction {
+    return new BooleanFunction(
+      new UnaryMetadata(this.metadata, IS_FALSY),
+      condition(this, false, (expr: boolean) => !Boolean(expr), true)
+    );
+  }
+
+  public isTruthy(): BooleanFunction {
+    return new BooleanFunction(
+      new UnaryMetadata(this.metadata, IS_TRUTHY),
+      condition(this, false, (expr: boolean) => Boolean(expr), false)
+    );
+  }
+
   public not(): BooleanFunction {
     return new BooleanFunction(
       new UnaryMetadata(this.metadata, NOT),
diff --git a/src/dsl/lang/DefaultOperators.ts b/src/dsl/lang/DefaultOperators.ts
index 6e54c9e..089b183 100644
--- a/src/dsl/lang/DefaultOperators.ts
+++ b/src/dsl/lang/DefaultOperators.ts
@@ -6,6 +6,8 @@ export const VALIDATE: Operator = { readable: 'validate' };
 export const NOT: Operator = { readable: 'not' };
 export const AND: Operator = { readable: 'and' };
 export const OR: Operator = { readable: 'or' };
+export const IS_FALSY: Operator = { readable: 'is falsy' };
+export const IS_TRUTHY: Operator = { readable: 'is truthy' };
 export const THEN: Operator = { readable: 'then' };
 export const ELSE: Operator = { readable: 'else' };
 export const FUNCTION: Operator = { readable: 'function' };
diff --git a/test/dsl/lang/BooleanFunction.test.ts b/test/dsl/lang/BooleanFunction.test.ts
index 0096708..86fe0e8 100644
--- a/test/dsl/lang/BooleanFunction.test.ts
+++ b/test/dsl/lang/BooleanFunction.test.ts
@@ -11,12 +11,14 @@ const falseFunction = DOOV.lift(BooleanFunction, false);
 const nullField = DOOV.lift(BooleanFunction, null as any);
 const trueField = DOOV.boolean(DOOV.field<boolean, Model>('user', 'b'));
 const undefinedField = DOOV.boolean(DOOV.field<boolean, Model>('user', 'a'));
+const falsyField = DOOV.boolean(DOOV.field<boolean, Model>('user', 'birth'));
 
 beforeEach(() => {
   model = new Model();
   user = new User(1);
   user.name = 'test';
   user.b = true;
+  user.birth = undefined;
   model.user = user;
 });
 
@@ -210,4 +212,25 @@ describe('boolean function with left null with short circuit', () => {
   it('not null', () => {
     expect(nullField.not().get(model, new DefaultContext(true))).toEqual(false);
   });
+
+  it('is falsy', () => {
+    expect(falsyField.isFalsy().get(model, new DefaultContext(true))).toEqual(true);
+    expect(
+      trueFunction
+        .not()
+        .isFalsy()
+        .get(model, new DefaultContext(true))
+    ).toEqual(true);
+  });
+
+  it('is truthy', () => {
+    const context = new DefaultContext(true);
+    expect(trueFunction.isTruthy().get(model, new DefaultContext(true))).toEqual(true);
+    expect(
+      falsyField
+        .not()
+        .isTruthy()
+        .get(model, context)
+    ).toEqual(true);
+  });
 });