Allow rendering properties of primitive types that are not objects (#618)

* prevent value from being returned by Context.prototype.lookup if lookupHit is false

* add test for renderability of Array.length via dot notation

* Remove `typeof obj === 'object'` constraint in prop lookup

Allows rendering properties of primitive types that are not objects, such as a string.

* pop lookup needs to use hasOwnProperty for non-objs

* re-add  constraint in prop lookup, but make property lookups for primitives possible through dot notation

* add test to address #589 specifically

* enhance readability of primitiveHasOwnProperty and add comments to explain why it is used in one case but not the other

Author: raymond-lam

mustache.js

@@ -45,6 +45,19 @@
     return obj != null && typeof obj === 'object' && (propName in obj);
   }
 
+  /**
+   * Safe way of detecting whether or not the given thing is a primitive and
+   * whether it has the given property
+   */
+  function primitiveHasOwnProperty (primitive, propName) {  
+    return (
+      primitive != null
+      && typeof primitive !== 'object'
+      && primitive.hasOwnProperty
+      && primitive.hasOwnProperty(propName)
+    );
+  }
+
   // Workaround for https://issues.apache.org/jira/browse/COUCHDB-577
   // See https://github.com/janl/mustache.js/issues/189
   var regExpTest = RegExp.prototype.test;
@@ -377,11 +390,11 @@
     if (cache.hasOwnProperty(name)) {
       value = cache[name];
     } else {
-      var context = this, names, index, lookupHit = false;
+      var context = this, intermediateValue, names, index, lookupHit = false;
 
       while (context) {
         if (name.indexOf('.') > 0) {
-          value = context.view;
+          intermediateValue = context.view;
           names = name.split('.');
           index = 0;
 
@@ -395,20 +408,51 @@
            *
            * This is specially necessary for when the value has been set to
            * `undefined` and we want to avoid looking up parent contexts.
+           *
+           * In the case where dot notation is used, we consider the lookup
+           * to be successful even if the last "object" in the path is
+           * not actually an object but a primitive (e.g., a string, or an
+           * integer), because it is sometimes useful to access a property
+           * of an autoboxed primitive, such as the length of a string.
            **/
-          while (value != null && index < names.length) {
+          while (intermediateValue != null && index < names.length) {
             if (index === names.length - 1)
-              lookupHit = hasProperty(value, names[index]);
+              lookupHit = (
+                hasProperty(intermediateValue, names[index]) 
+                || primitiveHasOwnProperty(intermediateValue, names[index])
+              );
 
-            value = value[names[index++]];
+            intermediateValue = intermediateValue[names[index++]];
           }
         } else {
-          value = context.view[name];
+          intermediateValue = context.view[name];
+
+          /**
+           * Only checking against `hasProperty`, which always returns `false` if
+           * `context.view` is not an object. Deliberately omitting the check
+           * against `primitiveHasOwnProperty` if dot notation is not used.
+           *
+           * Consider this example:
+           * ```
+           * Mustache.render("The length of a football field is {{#length}}{{length}}{{/length}}.", {length: "100 yards"})
+           * ```
+           *
+           * If we were to check also against `primitiveHasOwnProperty`, as we do
+           * in the dot notation case, then render call would return:
+           *
+           * "The length of a football field is 9."
+           *
+           * rather than the expected:
+           *
+           * "The length of a football field is 100 yards."
+           **/
           lookupHit = hasProperty(context.view, name);
         }
 
-        if (lookupHit)
+        if (lookupHit) {
+          value = intermediateValue;
           break;
+        }
 
         context = context.parent;
       }

test/_files/dot_notation.js

@@ -19,5 +19,6 @@
   truthy: {
     zero: 0,
     notTrue: false
-  }
+  },
+  singletonList: [{singletonItem: "singleton item"}]
 })

test/_files/dot_notation.mustache

@@ -7,3 +7,6 @@
 <h2>Test truthy false values:</h2>
 <p>Zero: {{truthy.zero}}</p>
 <p>False: {{truthy.notTrue}}</p>
+<p>length of string should be rendered: {{price.currency.name.length}}</p>
+<p>length of string in a list should be rendered: {{#singletonList}}{{singletonItem.length}}{{/singletonList}}</p>
+<p>length of an array should be rendered: {{authors.length}}</p>

test/_files/dot_notation.txt

@@ -7,3 +7,6 @@
 <h2>Test truthy false values:</h2>
 <p>Zero: 0</p>
 <p>False: false</p>
+<p>length of string should be rendered: 3</p>
+<p>length of string in a list should be rendered: 14</p>
+<p>length of an array should be rendered: 2</p>