AbsolutePaths: written utility function popIfEndsWith()

It will be necessary in the next commit to evaluate the Etherpad base
install path.

src/node/utils/AbsolutePaths.js

@@ -19,5 +19,33 @@
  */
 
 var log4js = require('log4js');
+var _ = require('underscore');
 
 var absPathLogger = log4js.getLogger('AbsolutePaths');
+
+/**
+ * If stringArray's last elements are exactly equal to lastDesiredElements,
+ * returns a copy in which those last elements are popped, or false otherwise.
+ *
+ * @param {string[]} stringArray - The input array.
+ * @param {string[]} lastDesiredElements - The elements to remove from the end
+ *                   of the input array.
+ * @return {string[]|boolean} The shortened array, or false if there was no
+ *                            overlap.
+ */
+var popIfEndsWith = function(stringArray, lastDesiredElements) {
+  if (stringArray.length <= lastDesiredElements.length) {
+    absPathLogger.debug(`In order to pop "${lastDesiredElements.join(path.sep)}" from "${stringArray.join(path.sep)}", it should contain at least ${lastDesiredElements.length + 1 } elements`);
+
+    return false;
+  }
+
+  const lastElementsFound = _.last(stringArray, lastDesiredElements.length);
+
+  if (_.isEqual(lastElementsFound, lastDesiredElements)) {
+    return _.initial(stringArray, lastDesiredElements.length);
+  }
+
+  absPathLogger.debug(`${stringArray.join(path.sep)} does not end with "${lastDesiredElements.join(path.sep)}"`);
+  return false;
+};