diff --git a/src/index.js b/src/index.js
index 58f4b16..a1f7f89 100644
--- a/src/index.js
+++ b/src/index.js
@@ -34,6 +34,24 @@ export const assertEq = (a, b, message = `assertion error: ${a} !== ${b}`) => {
         throw new AssertionError(message)
     }
 }
+const test = (re, str = '') => re instanceof RegExp ? re.test(str) : new RegExp(re).test(str)
+
+export const assertErr = (f, err) => {
+    try {
+        f()
+    } catch (e) {
+        if (typeof err === 'string' || err instanceof RegExp) {
+            assert(test(err, e.message), `${err} did not match ${e.message}`)
+        } else if (typeof err === 'function') {
+            assert(e.constructor === err)
+        }
+
+        return
+    }
+
+    const expected = err ? `"${err}" ` : ""
+    assert(false, `expecting error ${expected}but function did not throw`)
+}
 
 export const assertCallback = () => {
     let result = false