fixed #249 - preserve variable value types with edge cases

(empty string, Boolean false)

history.txt

@@ -2015,4 +2015,5 @@ ______
 * Localization and Portuguese translation updates, thanks, Manuel!
 * Catalan translation update, thanks, Bernat!!
 * Threads: Text comparisons are now case-sensitive („fixes“ #175)
-* Threads: fixed # 179 - don’t identify primitive (static) C-Slots as implicit formal parameters
+* Threads: fixed #179 - don’t identify primitive (static) C-Slots as implicit formal parameters
+* Threads: fixed #249 - preserve variable value types with edge cases (empty string, Boolean false)

threads.js

@@ -2954,7 +2954,9 @@ VariableFrame.prototype.getVar = function (name, upvars) {
 };
 
 VariableFrame.prototype.addVar = function (name, value) {
-    this.vars[name] = (value === 0 ? 0 : value || null);
+    this.vars[name] = (value === 0 ? 0
+              : value === false ? false
+                       : value === '' ? '' : value || null);
 };
 
 VariableFrame.prototype.deleteVar = function (name) {