Add ensure array lib function to allocate/resize

Author: rreusser

src/lib/ensure_array.js

@@ -0,0 +1,27 @@
+/**
+* Copyright 2012-2017, Plotly, Inc.
+* All rights reserved.
+*
+* This source code is licensed under the MIT license found in the
+* LICENSE file in the root directory of this source tree.
+*/
+
+'use strict';
+
+/*
+ * Ensures an array has the right amount of storage space. If it doesn't
+ * exist, it creates an array. If it does exist, it returns it if too
+ * short or truncates it in-place.
+ *
+ * The goal is to just reuse memory to avoid a bit of excessive garbage
+ * collection.
+ */
+module.exports = function ensureArray(out, n) {
+    if(!Array.isArray(out)) out = [];
+
+    // If too long, truncate. (If too short, it will grow
+    // automatically so we don't care about that case)
+    out.length = n;
+
+    return out;
+};

src/lib/index.js

@@ -19,6 +19,7 @@ lib.isArray = require('./is_array');
 lib.mod = require('./mod');
 lib.toLogRange = require('./to_log_range');
 lib.relinkPrivateKeys = require('./relink_private');
+lib.ensureArray = require('./ensure_array');
 
 var coerceModule = require('./coerce');
 lib.valObjects = coerceModule.valObjects;