diff --git a/packages/common/src/guard.test.ts b/packages/common/src/guard.test.ts
new file mode 100644
index 0000000..a208961
--- /dev/null
+++ b/packages/common/src/guard.test.ts
@@ -0,0 +1,60 @@
+import {guard, isNumber, isString, isDefined, isUndefined, isNull} from './guard'
+
+function getNumber(n: number | null): number | null {
+  return n
+}
+
+function getString(str: string | undefined | null): string | undefined | null {
+  return str
+}
+
+describe('guard', () => {
+
+  describe('converts T | undefined | null to T', () => {
+    describe('isDefined', () => {
+      it('converts T | undefined | null to T', () => {
+        const s1: string | undefined | null = getString('test')
+        const s2: string = guard<string>(isDefined, s1)
+        expect(s2).toBe('test')
+      })
+    })
+
+    describe('isString', () => {
+      it('converts string | undefined | null to string', () => {
+        const s1: string | undefined | null = getString('test')
+        const s2: string = guard(isString, s1)
+        expect(s2).toBe('test')
+      })
+    })
+
+    describe('isNumber', () => {
+      it('converts number | undefined | null to number', () => {
+        const n1: number | undefined | null = getNumber(1)
+        const n2: number = guard(isNumber, n1)
+        expect(n2).toBe(1)
+      })
+    })
+
+    describe('isUndefined', () => {
+      const s1: string | undefined | null = getString(undefined)
+      const s2: undefined = guard(isUndefined, s1)
+      expect(s2).toBe(undefined)
+    })
+
+    describe('isNull', () => {
+      const s1: string | undefined | null = getString(null)
+      const s2: null = guard(isNull, s1)
+      expect(s2).toBe(null)
+    })
+
+  })
+
+  describe('errors', () => {
+    it('guards against unexpected types', () => {
+      const s1: string | undefined | null = getString(null)
+      expect(() => guard(isString, s1))
+      .toThrowError('Value null does not satisfy constraint: isString')
+    })
+  })
+
+})
diff --git a/packages/common/src/guard.ts b/packages/common/src/guard.ts
new file mode 100644
index 0000000..f6b6956
--- /dev/null
+++ b/packages/common/src/guard.ts
@@ -0,0 +1,35 @@
+type CheckType<T> = (t: unknown) => t is T
+
+export function guard<Expected>(
+  checkType: CheckType<Expected>,
+  t: unknown,
+): Expected {
+  if (!checkType(t)) {
+    throw new Error(
+      `Value ${t} does not satisfy constraint: ${checkType.name}`)
+  }
+
+  return t
+}
+
+export function isUndefined(t: unknown): t is undefined {
+  return t === undefined
+}
+
+export function isNull(t: unknown): t is null {
+  return t === null
+}
+
+export function isDefined<T>(
+  t: unknown,
+): t is T {
+  return t !== undefined && t !== null
+}
+
+export function isNumber(t: unknown): t is number {
+  return typeof t === 'number'
+}
+
+export function isString(t: unknown): t is string {
+  return typeof t === 'string'
+}
diff --git a/packages/common/src/index.ts b/packages/common/src/index.ts
index d1ce5c8..d7a101b 100644
--- a/packages/common/src/index.ts
+++ b/packages/common/src/index.ts
@@ -6,6 +6,7 @@ export * from './ILogger'
 export * from './IRole'
 export * from './StringUtils'
 export * from './filterProps'
+export * from './guard'
 export * from './indexBy'
 export * from './types'
 export * from './without'