Use headersSent instead of _header

HISTORY.md

@@ -1,3 +1,7 @@
+unreleased
+==========
+  * Use `res.headersSent` when available
+
 1.7.5 / 2024-10-31
 ==========
 

index.js

@@ -80,7 +80,7 @@ function compression (options) {
         return false
       }
 
-      if (!this._header) {
+      if (!headersSent(res)) {
         this._implicitHeader()
       }
 
@@ -94,7 +94,7 @@ function compression (options) {
         return false
       }
 
-      if (!this._header) {
+      if (!headersSent(res)) {
         // estimate the length
         if (!this.getHeader('Content-Length')) {
           length = chunkLength(chunk, encoding)
@@ -281,3 +281,17 @@ function toBuffer (chunk, encoding) {
     ? chunk
     : Buffer.from(chunk, encoding)
 }
+
+/**
+ * Determine if the response headers have been sent.
+ *
+ * @param {object} res
+ * @returns {boolean}
+ * @private
+ */
+
+function headersSent (res) {
+  return typeof res.headersSent !== 'boolean'
+    ? Boolean(res._header)
+    : res.headersSent
+}