buffer: add constants object

Add `buffer.constants`, containing length limits for `Buffer` and
`string` instances.

This could be useful for programmers to tell whether a value can
be turned into a string or not.

Ref: https://github.com/nodejs/node/issues/13465
PR-URL: https://github.com/nodejs/node/pull/13467
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/buffer.js b/lib/buffer.js
index f6ac8cbb81..1db4527364 100644
--- a/lib/buffer.js
+++ b/lib/buffer.js
@@ -42,8 +42,31 @@ Buffer.prototype = FastBuffer.prototype;
 exports.Buffer = Buffer;
 exports.SlowBuffer = SlowBuffer;
 exports.INSPECT_MAX_BYTES = 50;
+
+// Legacy.
 exports.kMaxLength = binding.kMaxLength;
 
+const constants = Object.defineProperties({}, {
+  MAX_LENGTH: {
+    value: binding.kStringMaxLength,
+    writable: false,
+    enumerable: true
+  },
+  MAX_STRING_LENGTH: {
+    value: binding.kStringMaxLength,
+    writable: false,
+    enumerable: true
+  }
+});
+
+Object.defineProperty(exports, 'constants', {
+  configurable: false,
+  enumerable: true,
+  value: constants
+});
+
+exports.kStringMaxLength = binding.kStringMaxLength;
+
 const kFromErrorMsg = 'First argument must be a string, Buffer, ' +
                       'ArrayBuffer, Array, or array-like object.';