feat: add decodeBase64 handlebars helper (renovatebot#36660)

Co-authored-by: Michael Kriese <michael.kriese@gmx.de>

Author: tenstad

docs/usage/templates.md

@@ -45,6 +45,14 @@ Returns `true` if a given string is a substring.
 
 `{{#if (containsString depName 'python')}}Python{{else}}Other{{/if}}`
 
+### decodeBase64
+
+If you want to convert a Base64 value to a string, use the built-in function `decodeBase64` like this:
+
+`{{{decodeBase64 body}}}`
+
+In the example above `body` is the base64 encoded value you want to decode.
+
 ### decodeURIComponent
 
 If you want to decode a percent-encoded string, use the built-in function `decodeURIComponent` like this:

lib/util/template/index.spec.ts

@@ -294,6 +294,30 @@ describe('util/template/index', () => {
     });
   });
 
+  describe('base64 decoding', () => {
+    it('decode values', () => {
+      const output = template.compile(
+        '{{{decodeBase64 "QGZzb3V6YS9wcmV0dGllcmQ="}}}',
+        undefined as never,
+      );
+      expect(output).toBe('@fsouza/prettierd');
+    });
+
+    it('handles null values gracefully', () => {
+      const output = template.compile('{{{decodeBase64 packageName}}}', {
+        packageName: null,
+      });
+      expect(output).toBe('');
+    });
+
+    it('handles undefined values gracefully', () => {
+      const output = template.compile('{{{decodeBase64 packageName}}}', {
+        packageName: undefined,
+      });
+      expect(output).toBe('');
+    });
+  });
+
   describe('equals', () => {
     it('equals', () => {
       const output = template.compile(

lib/util/template/index.ts

@@ -18,6 +18,10 @@ handlebars.registerHelper('encodeBase64', (str: string) =>
   Buffer.from(str ?? '').toString('base64'),
 );
 
+handlebars.registerHelper('decodeBase64', (str: string) =>
+  Buffer.from(str ?? '', 'base64').toString(),
+);
+
 handlebars.registerHelper('stringToPrettyJSON', (input: string): string =>
   JSON.stringify(JSON.parse(input), null, 2),
 );