buffer: improve toJSON() performance

PR-URL: https://github.com/nodejs/node/pull/10895
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/buffers/buffer-tojson.js

@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  n: [1e4],
+  len: [0, 10, 256, 4 * 1024]
+});
+
+function main(conf) {
+  var n = +conf.n;
+  var buf = Buffer.allocUnsafe(+conf.len);
+
+  bench.start();
+  for (var i = 0; i < n; ++i)
+    buf.toJSON();
+  bench.end(n);
+}

lib/buffer.js

@@ -804,10 +804,14 @@ Buffer.prototype.write = function(string, offset, length, encoding) {
 
 
 Buffer.prototype.toJSON = function() {
-  return {
-    type: 'Buffer',
-    data: Array.prototype.slice.call(this, 0)
-  };
+  if (this.length) {
+    const data = [];
+    for (var i = 0; i < this.length; ++i)
+      data[i] = this[i];
+    return { type: 'Buffer', data };
+  } else {
+    return { type: 'Buffer', data: [] };
+  }
 };