Bug 1260589 - Add the ThreadSafeDevToolsUtils.flatten utility; r=jimb a=kwierso

The `flatten` function takes an array of arrays and flattens them to a single
array, removing one level of nesting. It does not recursively flatten multiple
levels of nesting.

MozReview-Commit-ID: ErDMHbT2IX

--HG--
extra : histedit_source : eb6e4f8c54f536c84b08da22ef60cfa56a5075c8

devtools/shared/ThreadSafeDevToolsUtils.js

@@ -243,3 +243,14 @@ exports.isPromise = function (p) {
 exports.isSavedFrame = function (thing) {
   return Object.prototype.toString.call(thing) === "[object SavedFrame]";
 };
+
+/**
+ * Given a list of lists, flatten it. Only flattens one level; does not
+ * recursively flatten all levels.
+ *
+ * @param {Array<Array<Any>>} lists
+ * @return {Array<Any>}
+ */
+exports.flatten = function(lists) {
+  return Array.prototype.concat.apply([], lists);
+};

devtools/shared/tests/unit/test_flatten.js

@@ -0,0 +1,24 @@
+/* -*- js-indent-level: 2; indent-tabs-mode: nil -*- */
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Test ThreadSafeDevToolsUtils.flatten
+
+function run_test() {
+  const { flatten } = DevToolsUtils;
+
+  const flat = flatten([["a", "b", "c"],
+                        ["d", "e", "f"],
+                        ["g", "h", "i"]]);
+
+  equal(flat.length, 9);
+  equal(flat[0], "a");
+  equal(flat[1], "b");
+  equal(flat[2], "c");
+  equal(flat[3], "d");
+  equal(flat[4], "e");
+  equal(flat[5], "f");
+  equal(flat[6], "g");
+  equal(flat[7], "h");
+  equal(flat[8], "i");
+}

devtools/shared/tests/unit/xpcshell.ini

@@ -12,6 +12,7 @@ support-files =
 [test_fetch-file.js]
 [test_fetch-http.js]
 [test_fetch-resource.js]
+[test_flatten.js]
 [test_indentation.js]
 [test_independent_loaders.js]
 [test_invisible_loader.js]