Allow passing in buffer to finish

index.d.ts

@@ -2661,9 +2661,10 @@ export class Writer {
 
     /**
      * Finishes the write operation.
+     * @param {Uint8Array} [buf] Optional buffer that's 'this.len' bytes long
      * @returns Finished buffer
      */
-    public finish(): Uint8Array;
+    public finish(buf?: Uint8Array): Uint8Array;
 }
 
 /** Wire format writer using node buffers. */
@@ -2681,9 +2682,10 @@ export class BufferWriter extends Writer {
 
     /**
      * Finishes the write operation.
+     * @param {Buffer} [buf] Optional buffer that's 'this.len' bytes long
      * @returns Finished buffer
      */
-    public finish(): Buffer;
+    public finish(buf?: Buffer): Buffer;
 }
 
 /**

src/writer.js

@@ -443,18 +443,20 @@ Writer.prototype.ldelim = function ldelim() {
 
 /**
  * Finishes the write operation.
+ * @param {Uint8Array} [buf] Optional buffer that's 'this.len' bytes long
  * @returns {Uint8Array} Finished buffer
  */
-Writer.prototype.finish = function finish() {
+Writer.prototype.finish = function finish(buf) {
+    if (buf == null) {
+      buf = this.constructor.alloc(this.len);
+    }
     var head = this.head.next, // skip noop
-        buf  = this.constructor.alloc(this.len),
         pos  = 0;
     while (head) {
         head.fn(head.val, buf, pos);
         pos += head.len;
         head = head.next;
     }
-    // this.head = this.tail = null;
     return buf;
 };