Add FileReaderSync() documentation

files/en-us/web/api/filereader/index.md

@@ -20,7 +20,7 @@ File objects may be obtained from a {{domxref("FileList")}} object returned as a
 ## Constructor
 
 - {{domxref("FileReader.FileReader", "FileReader()")}}
-  - : Returns a newly constructed `FileReader`.
+  - : Returns a new `FileReader` object.
 
 See [Using files from web applications](/en-US/docs/Web/API/File_API/Using_files_from_web_applications) for details and examples.
 

files/en-us/web/api/filereadersync/filereadersync/index.md

@@ -0,0 +1,40 @@
+---
+title: FileReaderSync()
+slug: Web/API/FileReaderSync/FileReaderSync
+page-type: web-api-constructor
+browser-compat: api.FileReaderSync.FileReaderSync
+---
+
+{{APIRef("File API")}}
+
+The **`FileReaderSync()`** constructor creates a new {{domxref("FileReaderSync")}}.
+
+## Syntax
+
+```js-nolint
+new FileReaderSync()
+```
+
+### Parameters
+
+None.
+
+## Examples
+
+The following code snippet shows creation of a [`FileReaderSync`](/en-US/docs/Web/API/FileReaderSync) object using the `FileReaderSync()` constructor and subsequent usage of the object:
+
+```js
+function readFile(blob) {
+  const reader = new FileReaderSync();
+  postMessage(reader.readAsDataURL(blob));
+}
+```
+> **Note:** This snippet must be used inside a {{domxref("Worker")}}, as synchronous interfaces can't be used on the main thread.
+
+## Specifications
+
+{{Specifications}}
+
+## Browser compatibility
+
+{{Compat}}

files/en-us/web/api/filereadersync/index.md

@@ -13,6 +13,11 @@ The `FileReaderSync` interface allows to read {{DOMxRef("File")}} or {{DOMxRef("
 
 > **Warning:** This interface is **only available** in [workers](/en-US/docs/Web/API/Worker) as it enables synchronous I/O that could potentially block.
 
+## Constructor
+
+- {{domxref("FileReaderSync.FileReaderSync", "FileReaderSync()")}}
+  - : Returns a new `FileReaderSync` object.
+
 ## Instance properties
 
 This interface does not have any properties.