perf: optimize consumeEnd (nodejs#2510)

lib/api/readable.js

@@ -6,9 +6,7 @@ const assert = require('assert')
 const { Readable } = require('stream')
 const { RequestAbortedError, NotSupportedError, InvalidArgumentError, AbortError } = require('../core/errors')
 const util = require('../core/util')
-const { ReadableStreamFrom, toUSVString } = require('../core/util')
-
-let Blob
+const { ReadableStreamFrom } = require('../core/util')
 
 const kConsume = Symbol('kConsume')
 const kReading = Symbol('kReading')
@@ -267,14 +265,35 @@ function consumeStart (consume) {
   }
 }
 
+/**
+ * @param {Buffer[]} chunks
+ * @param {number} length
+ */
+function chunksDecode (chunks, length) {
+  if (chunks.length === 0 || length === 0) {
+    return ''
+  }
+  const buffer = chunks.length === 1 ? chunks[0] : Buffer.concat(chunks, length)
+
+  const start =
+   buffer.length >= 3 &&
+   // Skip BOM.
+      buffer[0] === 0xef &&
+      buffer[1] === 0xbb &&
+      buffer[2] === 0xbf
+     ? 3
+     : 0
+  return buffer.utf8Slice(start, buffer.length - start)
+}
+
 function consumeEnd (consume) {
   const { type, body, resolve, stream, length } = consume
 
   try {
     if (type === 'text') {
-      resolve(toUSVString(Buffer.concat(body)))
+      resolve(chunksDecode(body, length))
     } else if (type === 'json') {
-      resolve(JSON.parse(Buffer.concat(body)))
+      resolve(JSON.parse(chunksDecode(body, length)))
     } else if (type === 'arrayBuffer') {
       const dst = new Uint8Array(length)
 
@@ -286,9 +305,6 @@ function consumeEnd (consume) {
 
       resolve(dst.buffer)
     } else if (type === 'blob') {
-      if (!Blob) {
-        Blob = require('buffer').Blob
-      }
       resolve(new Blob(body, { type: stream[kContentType] }))
     }