Align with type declaration of template filter/global function by supporting undefined as return value.

Signed-off-by: Matt Benson <gudnabrsam@gmail.com>

.changeset/ninety-shirts-float.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+Align with type declaration of template filter/global function by supporting undefined as return value.

plugins/scaffolder-backend/src/lib/templating/SecureTemplater.test.ts

@@ -107,8 +107,9 @@ describe('SecureTemplater', () => {
     const mockFilter1 = jest.fn(() => 'filtered text');
     const mockFilter2 = jest.fn((var1, var2) => `${var1} ${var2}`);
     const mockFilter3 = jest.fn((var1, var2) => ({ var1, var2 }));
+    const mockFilter4 = jest.fn(() => undefined);
     const renderWith = await SecureTemplater.loadRenderer({
-      templateFilters: { mockFilter1, mockFilter2, mockFilter3 },
+      templateFilters: { mockFilter1, mockFilter2, mockFilter3, mockFilter4 },
     });
     const renderWithout = await SecureTemplater.loadRenderer();
 
@@ -131,6 +132,7 @@ describe('SecureTemplater', () => {
         var2: 'another extra arg',
       }),
     );
+    expect(renderWith('${{ inputValue | mockFilter4 }}', ctx)).toBe('');
 
     expect(() => renderWithout('${{ inputValue | mockFilter1 }}', ctx)).toThrow(
       /Error: filter not found: mockFilter1/,
@@ -152,8 +154,9 @@ describe('SecureTemplater', () => {
     const mockGlobal1 = jest.fn(() => 'awesome global function');
     const mockGlobal2 = 'foo';
     const mockGlobal3 = 123456;
+    const mockGlobal4 = jest.fn(() => undefined);
     const renderWith = await SecureTemplater.loadRenderer({
-      templateGlobals: { mockGlobal1, mockGlobal2, mockGlobal3 },
+      templateGlobals: { mockGlobal1, mockGlobal2, mockGlobal3, mockGlobal4 },
     });
     const renderWithout = await SecureTemplater.loadRenderer();
 
@@ -164,6 +167,7 @@ describe('SecureTemplater', () => {
     );
     expect(renderWith('${{ mockGlobal2 }}', ctx)).toBe('foo');
     expect(renderWith('${{ mockGlobal3 }}', ctx)).toBe('123456');
+    expect(renderWith('${{ mockGlobal4() }}', ctx)).toBe('');
 
     expect(() => renderWithout('${{ mockGlobal1() }}', ctx)).toThrow(
       /Error: Unable to call `mockGlobal1`/,

plugins/scaffolder-backend/src/lib/templating/SecureTemplater.ts

@@ -52,14 +52,17 @@ const { render, renderCompat } = (() => {
   });
   compatEnv.addFilter('jsonify', compatEnv.getFilter('dump'));
 
+  const handleFunctionResult = (value) => {
+    return value === '' ? undefined : JSON.parse(value);
+  };
   for (const name of JSON.parse(availableTemplateFilters)) {
-    env.addFilter(name, (...args) => JSON.parse(callFilter(name, args)));
+    env.addFilter(name, (...args) => handleFunctionResult(callFilter(name, args)));
   }
   for (const [name, value] of Object.entries(JSON.parse(availableTemplateGlobals))) {
     env.addGlobal(name, value);
   }
   for (const name of JSON.parse(availableTemplateCallbacks)) {
-    env.addGlobal(name, (...args) => JSON.parse(callGlobal(name, args)));
+    env.addGlobal(name, (...args) => handleFunctionResult(callGlobal(name, args)));
   }
 
   let uninstallCompat = undefined;
@@ -187,7 +190,8 @@ export class SecureTemplater {
         if (!Object.hasOwn(templateFilters, filterName)) {
           return '';
         }
-        return JSON.stringify(templateFilters[filterName](...args));
+        const rz = templateFilters[filterName](...args);
+        return rz === undefined ? '' : JSON.stringify(rz);
       },
     );
 
@@ -201,7 +205,8 @@ export class SecureTemplater {
         if (typeof global !== 'function') {
           return '';
         }
-        return JSON.stringify(global(...args));
+        const rz = global(...args);
+        return rz === undefined ? '' : JSON.stringify(rz);
       },
     );