diff --git a/src/webidl/domexception.ts b/src/domexception.ts
similarity index 100%
rename from src/webidl/domexception.ts
rename to src/domexception.ts
diff --git a/src/file.ts b/src/file.ts
new file mode 100644
index 0000000..ec7ac58
--- /dev/null
+++ b/src/file.ts
@@ -0,0 +1,66 @@
+/**
+ * @license
+ * Copyright (c) 2023 Yamagishi Kazutoshi
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+import { Blob, type BlobOptions } from 'buffer';
+import { type BinaryLike } from 'crypto';
+
+/**
+ * @see {@link https://w3c.github.io/FileAPI/#typedefdef-blobpart File API - typedef (BufferSource or Blob or USVString) BlobPart}
+ */
+export type BlobPart = BinaryLike | Blob;
+
+/**
+ * @see {@link https://w3c.github.io/FileAPI/#dfn-FilePropertyBag File API - interface FilePropertyBag}
+ */
+export interface FilePropertyBag extends BlobOptions {
+  lastModified?: number;
+}
+
+/**
+ * @see {@link https://w3c.github.io/FileAPI/#dfn-file File API - interface File}
+ */
+export default class File extends Blob {
+  #lastModifiled: number;
+  #name: string;
+
+  constructor(
+    fileBits: BlobPart[],
+    fileName: string,
+    options?: FilePropertyBag
+  ) {
+    const { lastModified = Date.now(), ...blobPropertyBag } = options ?? {};
+
+    super(fileBits, blobPropertyBag);
+
+    this.#name = fileName;
+    this.#lastModifiled = lastModified;
+  }
+
+  get lastModified(): number {
+    return this.#lastModifiled;
+  }
+
+  get name(): string {
+    return this.#name;
+  }
+}
diff --git a/src/formdata.ts b/src/formdata.ts
index 8a43d25..7204084 100644
--- a/src/formdata.ts
+++ b/src/formdata.ts
@@ -21,14 +21,103 @@
  * THE SOFTWARE.
  */
 
+import { Blob } from 'buffer';
+import File from './file';
+
+/**
+ * @see {@link https://xhr.spec.whatwg.org/#formdataentryvalue XMLHttpRequest Standard - typedef (File or USVString) FormDataEntryValue}
+ */
+export type FormDataEntryValue = File | string;
+
 /**
  * @see {@link https://xhr.spec.whatwg.org/#interface-formdata XMLHttpRequest Standard - 5. Interface FormData}
  */
 export default class FormData {
+  #entryList = new Map<string, Set<FormDataEntryValue>>();
+
+  /**
+   * @see {@link https://xhr.spec.whatwg.org/#dom-formdata-append XMLHttpRequest Standard - The append(name, value) and append(name, blobValue, filename) method}
+   */
+  append(name: string, value: string): void;
+  append(name: string, blobValue: Blob, filename?: string): void;
+  append(name: string, value: Blob | string, ...args: string[]): void {
+    let entry: FormDataEntryValue;
+
+    if (value instanceof Blob) {
+      const filename = args[0] ?? (value instanceof File ? value.name : 'blob');
+      entry = new File([value], filename, { type: value.type });
+    } else {
+      entry = value;
+    }
+
+    const entrySet = this.#entryList.get(name);
+
+    if (entrySet) {
+      entrySet.add(entry);
+    } else {
+      this.#entryList.set(name, new Set<FormDataEntryValue>([entry]));
+    }
+  }
+
   /**
-   * @todo Implement this function.
+   * @see {@link https://xhr.spec.whatwg.org/#dom-formdata-delete XMLHttpRequest Standard - The delete(name) method}
    */
-  append(/* name, value, filename */): void {
-    // wip
+  delete(name: string): void {
+    this.#entryList.delete(name);
+  }
+
+  /**
+   * @see {@link https://xhr.spec.whatwg.org/#dom-formdata-get XMLHttpRequest Standard - The get(name) method}
+   */
+  get(name: string): FormDataEntryValue | undefined {
+    const entrySet = this.#entryList.get(name);
+
+    if (!entrySet) {
+      return;
+    }
+
+    return [...entrySet][0];
+  }
+
+  /**
+   * @see {@link https://xhr.spec.whatwg.org/#dom-get-getall XMLHttpRequest Standard - The getAll(name) method}
+   */
+  getAll(name: string): FormDataEntryValue[] {
+    const entrySet = this.#entryList.get(name);
+
+    return entrySet ? [...entrySet] : [];
+  }
+
+  /**
+   * @see {@link https://xhr.spec.whatwg.org/#dom-formdata-has XMLHttpRequest Standard - The has(name) method}
+   */
+  has(name: string): boolean {
+    return this.#entryList.has(name);
+  }
+
+  /**
+   * @see {@link https://xhr.spec.whatwg.org/#dom-formdata-set XMLHttpRequest Standard - The set(name, value) and set(name, blobValue, filename) method}
+   */
+  set(name: string, value: string): void;
+  set(name: string, blobValue: Blob, filename: string): void;
+  set(name: string, value: Blob | string, ...args: string[]): void {
+    let entry: FormDataEntryValue;
+
+    if (value instanceof Blob) {
+      const filename = args[0] ?? (value instanceof File ? value.name : 'blob');
+      entry = new File([value], filename, { type: value.type });
+    } else {
+      entry = value;
+    }
+
+    this.#entryList.set(name, new Set<FormDataEntryValue>([entry]));
+  }
+
+  *[Symbol.iterator](): IterableIterator<[string, FormDataEntryValue]> {
+    for (const [name, entrySet] of this.#entryList.entries()) {
+      for (const entry of entrySet.values()) {
+        yield [name, entry];
+      }
+    }
   }
 }
diff --git a/src/index.ts b/src/index.ts
index 02ecb99..ad3efb6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -23,4 +23,5 @@
 
 export { default as XMLHttpRequest } from './xmlhttprequest';
 export { default as XMLHttpRequestUpload } from './xmlhttprequestupload';
+export { default as File } from './file';
 export { default as FormData } from './formdata';
diff --git a/src/xmlhttprequest.ts b/src/xmlhttprequest.ts
index bc5da21..ab94d7c 100644
--- a/src/xmlhttprequest.ts
+++ b/src/xmlhttprequest.ts
@@ -21,11 +21,13 @@
  * THE SOFTWARE.
  */
 
+import { Blob } from 'buffer';
 import * as http from 'http';
 import * as https from 'https';
+import DOMException from './domexception';
+import File from './file';
 import FormData from './formdata';
 import ProgressEvent from './progressevent';
-import DOMException from './webidl/domexception';
 import XMLHttpRequestEventTarget from './xmlhttprequesteventtarget';
 import XMLHttpRequestUpload from './xmlhttprequestupload';
 
@@ -77,12 +79,11 @@ const FORBIDDEN_RESPONSE_HEADERS = ['set-cookie', 'set-cookie2'];
  */
 const HTTP_HEADER_FIELD_NAME_REGEXP = /[!#$%&'*+-.^_`|~a-z0-9]+/;
 
-export type BodyInit =
-  | ArrayBuffer
-  | Buffer
+export type XMLHttpRequestBodyInit =
+  | Blob
+  | BufferSource
   | FormData
   | URLSearchParams
-  | Uint8Array
   | string;
 
 export type XMLHttpRequestResponseType =
@@ -478,24 +479,64 @@ export default class XMLHttpRequest extends XMLHttpRequestEventTarget {
   /**
    * @see {@link https://xhr.spec.whatwg.org/#the-send()-method XMLHttpRequest Standard - 4.5.6. The send() method}
    */
-  send(body: BodyInit | null = null): void {
+  send(body: XMLHttpRequestBodyInit | null = null): void {
     if (this.readyState !== XMLHttpRequest.OPENED || !this.#client) {
       // TODO: Add human readable message.
       throw new DOMException('', 'InvalidStateError');
     }
 
-    if (body) {
-      const bodyInit =
-        body instanceof ArrayBuffer || body instanceof Uint8Array
-          ? Buffer.from(body)
-          : body;
+    this.dispatchEvent(new ProgressEvent('loadstart'));
+
+    if (body && !['GET', 'HEAD'].includes(this.#client.method)) {
+      let chunk = new Blob([]);
 
-      if (typeof bodyInit === 'string' || bodyInit instanceof Buffer) {
-        const length = Buffer.isBuffer(bodyInit)
-          ? bodyInit.length
-          : Buffer.byteLength(bodyInit);
+      if (
+        body instanceof ArrayBuffer ||
+        ArrayBuffer.isView(body) ||
+        body instanceof Blob
+      ) {
+        // eslint-disable-next-line @typescript-eslint/ban-ts-comment
+        // @ts-ignore
+        chunk = new Blob([body]);
+      } else if (body instanceof URLSearchParams) {
+        chunk = new Blob([body.toString()], {
+          type: 'application/x-www-form-urlencoded; charset=UTF-8'
+        });
+      } else if (body instanceof FormData) {
+        const boundary = '------xxxxx';
+
+        let chunk = new Blob([], {
+          type: `multipart/form-data; boundary=${boundary}`
+        });
+        for (const [name, value] of body) {
+          if (value instanceof File) {
+            chunk = new Blob(
+              [
+                chunk,
+                `Content-Disposition: form-data; name="${name}"; filename="${value.name}"\r\n`,
+                '\r\n',
+                value,
+                `\r\n`
+              ],
+              { type: chunk.type }
+            );
+          } else {
+            chunk = new Blob(
+              [
+                chunk,
+                `${boundary}\r\n`,
+                `Content-Disposition: form-data; name="${name}"\r\n`,
+                '\r\n',
+                `${value}\r\n`
+              ],
+              { type: chunk.type }
+            );
+          }
+        }
 
-        this.#client.setHeader('Content-Length', length);
+        chunk = new Blob([chunk, `${boundary}\r\n`], { type: chunk.type });
+      } else {
+        chunk = new Blob([body], { type: 'text/plain' });
       }
 
       this.#client.addListener('socket', (socket) => {
@@ -511,10 +552,26 @@ export default class XMLHttpRequest extends XMLHttpRequestEventTarget {
         });
       });
 
-      this.#client.write(body);
+      if (chunk.type) {
+        this.setRequestHeader('Content-Type', chunk.type);
+      }
+
+      this.setRequestHeader('Content-Length', chunk.size.toString());
+
+      chunk
+        .arrayBuffer()
+        .then((buffer) => {
+          if (!this.#client) {
+            throw new TypeError('The client is initialized unintentionally.');
+          }
+
+          this.#client.write(new Uint8Array(buffer));
+        })
+        .catch((error) => {
+          throw error;
+        });
     }
 
-    this.dispatchEvent(new ProgressEvent('loadstart'));
     this.#client.end();
   }
 
diff --git a/test/integration/formdata.ts b/test/integration/formdata.ts
new file mode 100644
index 0000000..6df6c30
--- /dev/null
+++ b/test/integration/formdata.ts
@@ -0,0 +1,70 @@
+/**
+ * @license
+ * Copyright (c) 2023 Yamagishi Kazutoshi
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+import { FormData } from '../..';
+
+describe('FormData', () => {
+  describe('.append()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+
+      expect(formData.append('message', 'test message')).toBeUndefined();
+    });
+  });
+
+  describe('.delete()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+
+  describe('.get()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+
+  describe('.getAll()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+
+  describe('.has()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+
+  describe('.set()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+
+  describe('.[Symbol.iterator]()', () => {
+    it('basic use case', () => {
+      const formData = new FormData();
+    });
+  });
+});
diff --git a/test/integration/xmlhttprequest.ts b/test/integration/xmlhttprequest.ts
index bcce5cd..4476908 100644
--- a/test/integration/xmlhttprequest.ts
+++ b/test/integration/xmlhttprequest.ts
@@ -21,6 +21,7 @@
  * THE SOFTWARE.
  */
 
+import { Blob } from 'buffer';
 import * as http from 'http';
 import getPort from 'get-port';
 import { XMLHttpRequest } from '../..';
@@ -33,17 +34,42 @@ const launchMockServer = (port: number, hostname = 'localhost') =>
   new Promise<http.Server>((resolve) => {
     const server = http.createServer((req, res) => {
       const url = new URL(req.url || '/', `http://${hostname}:${port}`);
-      const status = parseInt(url.searchParams.get('status') || '200', 10);
-      const type = url.searchParams.get('type') || 'text/plain';
-      const body = url.searchParams.get('body') || '';
-
-      res.writeHead(status, {
-        'Cache-Control': 'max-age=60',
-        'Content-Type': type,
-        Date: referenceTime.toUTCString()
+      const status = parseInt(url.searchParams.get('status') ?? '200', 10);
+      const type = url.searchParams.get('type') ?? 'text/plain';
+      const body = url.searchParams.get('body') ?? '';
+      const delay = parseInt(url.searchParams.get('timeout') ?? '0', 0);
+
+      let blob = new Blob([], {
+        type: req.headers['content-type']
+      });
+
+      req.addListener('data', (chunk: Buffer) => {
+        blob = new Blob([blob, chunk], {
+          type: blob.type
+        });
+      });
+
+      req.addListener('end', () => {
+        res.writeHead(status, {
+          'Cache-Control': 'max-age=60',
+          'Content-Type': type,
+          Date: referenceTime.toUTCString()
+        });
+
+        if (body) {
+          res.write(body);
+        } else if (blob.size > 0) {
+          res.write(blob);
+        }
+
+        if (delay > 0) {
+          setTimeout(() => {
+            res.end();
+          }, delay);
+        } else {
+          res.end();
+        }
       });
-      res.write(body);
-      res.end();
     });
 
     server.keepAliveTimeout = defaultKeepAliveTimeout;
@@ -309,7 +335,7 @@ describe('XMLHttpRequest', () => {
   });
 
   describe('.responseURL', () => {
-    it('', (done) => {
+    it('basic use case', (done) => {
       const client = new XMLHttpRequest();
 
       client.addEventListener('loadstart', () => {
@@ -327,6 +353,39 @@ describe('XMLHttpRequest', () => {
     });
   });
 
+  describe('.timeout', () => {
+    it('basic use case', (done) => {
+      const client = new XMLHttpRequest();
+
+      client.addEventListener('timeout', () => {
+        done();
+      });
+
+      client.open('GET', `${baseURL}/?delay=10000`);
+      client.timeout = 1_000;
+      client.send(null);
+    });
+  });
+
+  describe('.send()', () => {
+    it('send URLSearchParams', () => {
+      const client = new XMLHttpRequest();
+      const searchParams = new URLSearchParams();
+
+      searchParams.append('subject', 'test subject');
+      searchParams.append('message', 'value1');
+      searchParams.append('message', 'value2');
+      searchParams.append('message', 'value3');
+
+      client.addEventListener('load', () => {
+        expect(client.responseText).toEqual('test');
+      });
+
+      client.open('POST', `${baseURL}/`);
+      client.send(searchParams);
+    });
+  });
+
   describe('.withCredentials', () => {
     it('throws InvalidStateError when readyState is DONE', (done) => {
       const client = new XMLHttpRequest();