Refactoring representation of variable type.

src/engine/target.js

@@ -124,7 +124,7 @@ class Target extends EventEmitter {
         const list = this.lookupVariableById(id);
         if (list) return list;
         // No variable with this name exists - create it locally.
-        const newList = new Variable(id, name, 'list', false);
+        const newList = new Variable(id, name, Variable.LIST_TYPE, false);
         this.variables[id] = newList;
         return newList;
     }
@@ -134,7 +134,7 @@ class Target extends EventEmitter {
      * dictionary of variables.
      * @param {string} id Id of variable
      * @param {string} name Name of variable.
-     * @param {string} type Type of variable, one of string, number, list
+     * @param {string} type Type of variable, '' or 'list'
      */
     createVariable (id, name, type) {
         if (!this.variables.hasOwnProperty(id)) {

src/engine/variable.js

@@ -19,18 +19,36 @@ class Variable {
         this.type = type;
         this.isCloud = isCloud;
         switch (this.type) {
-        case '':
+        case Variable.SCALAR_TYPE:
             this.value = 0;
             break;
-        case 'list':
+        case Variable.LIST_TYPE:
             this.value = [];
             break;
+        default:
+            throw new Error(`Invalid variable type: ${this.type}`);
         }
     }
 
     toXML () {
         return `<variable type="${this.type}" id="${this.id}">${this.name}</variable>`;
     }
+
+    /**
+     * Type representation for scalar variables.
+     * @const {string}
+     */
+    static get SCALAR_TYPE () {
+        return '';
+    }
+
+    /**
+     * Type representation for list variables.
+     * @const {string}
+     */
+    static get LIST_TYPE () {
+        return 'list';
+    }
 }
 
 module.exports = Variable;