Add example implementation of readAll

Author: oleiade

docs/sources/next/javascript-api/k6-experimental/fs/file/read.md

@@ -22,6 +22,10 @@ A [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Gl
 
 ## Example
 
+### Reading a file
+
+In the following example, we open a file and read it in chunks of 128 bytes until we reach the end of the file.
+
 {{< code >}}
 
 ```javascript
@@ -33,13 +37,7 @@ let file;
 })();
 
 export default async function () {
-  // About information about the file
-  const fileinfo = await file.stat();
-  if (fileinfo.name != 'bonjour.txt') {
-    throw new Error('Unexpected file name');
-  }
-
-  const buffer = new Uint8Array(4);
+  const buffer = new Uint8Array(128);
 
   let totalBytesRead = 0;
   while (true) {
@@ -59,11 +57,67 @@ export default async function () {
     }
   }
 
-  // Check that we read the expected number of bytes
-  if (totalBytesRead != fileinfo.size) {
-    throw new Error('Unexpected number of bytes read');
+  // Seek back to the beginning of the file
+  await file.seek(0, SeekMode.Start);
+}
+```
+
+{{< /code >}}
+
+### `readAll` helper function
+
+The following helper function can be used to read the entire contents of a file into a [Uint8Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array) buffer.
+
+{{< code >}}
+
+```javascript
+import { open, SeekMode } from 'k6/experimental/fs';
+
+let file;
+(async function () {
+  file = await open('bonjour.txt');
+})();
+
+async function readAll(file, bufferSize = 128) {
+  // Obtain the size of the file
+  const fileInfo = await file.stat();
+  const fileSize = fileInfo.size;
+
+  // Prepare a buffer to store the file content
+  const fileContent = new Uint8Array(fileInfo.size);
+  let fileOffset = 0;
+
+  // Prepare a buffer to read the chunks of the file into
+  const buffer = new Uint8Array(Math.min(bufferSize, fileSize));
+
+  while (true) {
+    // Read a chunk of the file
+    const bytesRead = await file.read(buffer);
+    if (bytesRead == null || bytesRead === 0) {
+      // EOF
+      break;
+    }
+
+    // Copy the content of the buffer into the file content buffer
+    fileContent.set(buffer.slice(0, bytesRead), fileOffset);
+
+    // Do something useful with the content of the buffer
+    fileOffset += bytesRead;
+
+    // If bytesRead is less than the buffer size, we've read the whole file
+    if (bytesRead < buffer.byteLength) {
+      break;
+    }
   }
 
+  return fileContent;
+}
+
+export default async function () {
+  // Read the whole file
+  const fileContent = await readAll(file);
+  console.log(JSON.stringify(fileContent));
+
   // Seek back to the beginning of the file
   await file.seek(0, SeekMode.Start);
 }