fetch: add automatic request body compression via compress option

packages/bun-types/globals.d.ts

@@ -2034,6 +2034,37 @@ interface BunFetchRequestInit extends RequestInit {
    */
   decompress?: boolean;
 
+  /**
+   * Automatically compress the request body before sending and set the
+   * `Content-Encoding` request header accordingly.
+   *
+   * - `true` is equivalent to `"gzip"`.
+   * - A string selects the encoding with its default level.
+   * - An object selects the encoding and an explicit compression `level`.
+   *
+   * Only buffered bodies (string, `ArrayBuffer`/`TypedArray`, `Blob`) are
+   * compressed; `ReadableStream` bodies are sent as-is. If the request
+   * already has a `Content-Encoding` header, the body is left unchanged.
+   * This is a custom property that is not part of the Fetch API specification.
+   *
+   * @default false
+   * @example
+   * ```js
+   * await fetch("https://example.com/upload", {
+   *   method: "POST",
+   *   body: JSON.stringify(bigPayload),
+   *   compress: "gzip",
+   * });
+   * ```
+   */
+  compress?:
+    | boolean
+    | "gzip"
+    | "deflate"
+    | "br"
+    | "zstd"
+    | { encoding: "gzip" | "deflate" | "br" | "zstd"; level?: number };
+
   /**
    * The maximum number of redirects to follow when `redirect` is `"follow"`.
    * If the response chain redirects more than this many times, the request

src/runtime/webcore/fetch.rs

@@ -40,6 +40,9 @@ pub(crate) const FETCH_TYPE_ERROR_STRINGS: [&str; 8] = FETCH_TYPE_ERROR_STRING_V
 #[path = "fetch/FetchTasklet.rs"]
 pub mod fetch_tasklet;
 
+#[path = "fetch/compress_body.rs"]
+pub mod compress_body;
+
 // ──────────────────────────────────────────────────────────────────────────
 // fetch() implementation
 // ──────────────────────────────────────────────────────────────────────────
@@ -420,6 +423,7 @@ fn fetch_impl<const ALLOW_GET_BODY: bool>(
     let mut disable_timeout = false;
     let mut disable_keepalive = false;
     let mut disable_decompression = false;
+    let mut compress: Option<compress_body::CompressOption> = None;
     let mut max_redirects: Option<u8> = None;
     let mut verbose: http::HTTPVerboseLevel = if vm
         .log_ref()
@@ -664,6 +668,34 @@ fn fetch_impl<const ALLOW_GET_BODY: bool>(
         return Ok(JSValue::ZERO);
     }
 
+    // "compress: boolean | string | { encoding, level? }"
+    'extract_compress: {
+        let objects_to_try = [
+            options_object.unwrap_or(JSValue::ZERO),
+            request_init_object.unwrap_or(JSValue::ZERO),
+        ];
+
+        for obj in objects_to_try {
+            if !obj.is_empty() {
+                if let Some(compress_value) = obj.get(global_this, "compress")? {
+                    if !compress_value.is_undefined() {
+                        compress =
+                            compress_body::CompressOption::from_js(global_this, compress_value)?;
+                        break 'extract_compress;
+                    }
+                }
+
+                if global_this.has_exception() {
+                    return Ok(JSValue::ZERO);
+                }
+            }
+        }
+    }
+
+    if global_this.has_exception() {
+        return Ok(JSValue::ZERO);
+    }
+
     // "maxRedirects: number"
     'extract_max_redirects: {
         let objects_to_try = [
@@ -1731,6 +1763,35 @@ fn fetch_impl<const ALLOW_GET_BODY: bool>(
         }
     }
 
+    // Automatic request-body compression. Only buffered bodies (Blob bytes,
+    // ArrayBuffer/TypedArray, string) are handled; ReadableStream and sendfile
+    // are skipped. S3 destinations replace the header set with a signed one,
+    // so compression is skipped there too.
+    if let Some(compress_opt) = compress
+        && let HTTPRequestBody::AnyBlob(_) = &body
+        && !url.is_s3()
+    {
+        let already_has_encoding = headers
+            .as_ref()
+            .and_then(|h| h.get_content_encoding())
+            .is_some();
+        if !already_has_encoding {
+            let input = body.slice();
+            if !input.is_empty() {
+                let compressed =
+                    compress_body::compress_request_body(global_this, input, &compress_opt)?;
+                let mut old = core::mem::replace(
+                    &mut body,
+                    HTTPRequestBody::AnyBlob(blob::Any::from_owned_slice(compressed)),
+                );
+                old.detach();
+                headers
+                    .get_or_insert_default()
+                    .append(b"Content-Encoding", compress_opt.encoding.header_value());
+            }
+        }
+    }
+
     if url.is_s3() {
         // get ENV config — `Transpiler::env_mut` is the safe accessor for the
         // process-singleton dotenv loader (set during init).