diff --git a/types/index.d.ts b/types/index.d.ts
index 150571729..24a4f4e68 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -258,6 +258,8 @@ declare namespace math {
   }
   interface FunctionNodeCtor {
     new (fn: MathNode | string, args: MathNode[]): FunctionNode
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    onUndefinedFunction: (name: string) => any
   }
 
   interface IndexNode extends MathNode {
@@ -383,6 +385,8 @@ declare namespace math {
   }
   interface SymbolNodeCtor {
     new (name: string): SymbolNode
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    onUndefinedSymbol: (name: string) => any
   }
 
   /**
diff --git a/types/index.ts b/types/index.ts
index 451e35a2c..3eb6f8f3e 100644
--- a/types/index.ts
+++ b/types/index.ts
@@ -1488,6 +1488,19 @@ Units examples
   math.unit('1 m').splitUnit(['ft', 'in'])
 }
 
+/**
+ * Example of custom fallback for onUndefinedSymbol & onUndefinedFunction
+ */
+{
+  const math = create(all, {})
+
+  math.SymbolNode.onUndefinedSymbol = () => null
+
+  assert.strictEqual(math.evaluate('nonExistingSymbol'), null)
+
+  math.FunctionNode.onUndefinedFunction = () => () => 42
+}
+
 /*
 Expression tree examples
 */