doc: buffer.fill empty value

PR-URL: #45794
Fixes: #45727
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>

doc/api/buffer.md

@@ -1884,6 +1884,7 @@ changes:
 -->
 
 * `value` {string|Buffer|Uint8Array|integer} The value with which to fill `buf`.
+  Empty value (string, Uint8Array, Buffer) is coerced to `0`.
 * `offset` {integer} Number of bytes to skip before starting to fill `buf`.
   **Default:** `0`.
 * `end` {integer} Where to stop filling `buf` (not inclusive). **Default:**
@@ -1904,6 +1905,12 @@ const b = Buffer.allocUnsafe(50).fill('h');
 
 console.log(b.toString());
 // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
+
+// Fill a buffer with empty string
+const c = Buffer.allocUnsafe(5).fill('');
+
+console.log(c.fill(''));
+// Prints: <Buffer 00 00 00 00 00>
 ```
 
 ```cjs
@@ -1915,6 +1922,12 @@ const b = Buffer.allocUnsafe(50).fill('h');
 
 console.log(b.toString());
 // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
+
+// Fill a buffer with empty string
+const c = Buffer.allocUnsafe(5).fill('');
+
+console.log(c.fill(''));
+// Prints: <Buffer 00 00 00 00 00>
 ```
 
 `value` is coerced to a `uint32` value if it is not a string, `Buffer`, or

test/parallel/test-buffer-fill.js

@@ -429,3 +429,18 @@ assert.throws(() => {
   code: 'ERR_INVALID_ARG_VALUE',
   name: 'TypeError'
 });
+
+
+{
+  const bufEmptyString = Buffer.alloc(5, '');
+  assert.strictEqual(bufEmptyString.toString(), '\x00\x00\x00\x00\x00');
+
+  const bufEmptyArray = Buffer.alloc(5, []);
+  assert.strictEqual(bufEmptyArray.toString(), '\x00\x00\x00\x00\x00');
+
+  const bufEmptyBuffer = Buffer.alloc(5, Buffer.alloc(5));
+  assert.strictEqual(bufEmptyBuffer.toString(), '\x00\x00\x00\x00\x00');
+
+  const bufZero = Buffer.alloc(5, 0);
+  assert.strictEqual(bufZero.toString(), '\x00\x00\x00\x00\x00');
+}