Fix iteration over undefined values

Allow for iteration on undefined values, but special case undefined and null to prevent rendering errors when not running in strict mode.

Fixes #1093

lib/handlebars/compiler/javascript-compiler.js

@@ -984,13 +984,14 @@ JavaScriptCompiler.prototype = {
   setupHelper: function(paramSize, name, blockHelper) {
     let params = [],
         paramsInit = this.setupHelperArgs(name, paramSize, params, blockHelper);
-    let foundHelper = this.nameLookup('helpers', name, 'helper');
+    let foundHelper = this.nameLookup('helpers', name, 'helper'),
+        callContext = this.aliasable(`${this.contextName(0)} != null ? ${this.contextName(0)} : {}`);
 
     return {
       params: params,
       paramsInit: paramsInit,
       name: foundHelper,
-      callParams: [this.contextName(0)].concat(params)
+      callParams: [callContext].concat(params)
     };
   },
 

lib/handlebars/helpers/each.js

@@ -25,12 +25,6 @@ export default function(instance) {
     }
 
     function execIteration(field, index, last) {
-      // Don't iterate over undefined values since we can't execute blocks against them
-      // in non-strict (js) mode.
-      if (context[field] == null) {
-        return;
-      }
-
       if (data) {
         data.key = field;
         data.index = index;
@@ -51,7 +45,9 @@ export default function(instance) {
     if (context && typeof context === 'object') {
       if (isArray(context)) {
         for (let j = context.length; i < j; i++) {
-          execIteration(i, i, i === context.length - 1);
+          if (i in context) {
+            execIteration(i, i, i === context.length - 1);
+          }
         }
       } else {
         let priorKey;

spec/regressions.js

@@ -204,6 +204,24 @@ describe('Regressions', function() {
     shouldCompileTo('{{#each array}}{{@index}}{{.}}{{/each}}', {array: array}, '1foo3bar');
   });
 
+  it('GH-1093: Undefined helper context', function() {
+    var obj = {foo: undefined, bar: 'bat'};
+    var helpers = {
+      helper: function() {
+        // It's valid to execute a block against an undefined context, but
+        // helpers can not do so, so we expect to have an empty object here;
+        for (var name in this) {
+          if (this.hasOwnProperty(name)) {
+            return 'found';
+          }
+        }
+        return 'not';
+      }
+    };
+
+    shouldCompileTo('{{#each obj}}{{{helper}}}{{.}}{{/each}}', [{obj: obj}, helpers], 'notfoundbat');
+  });
+
   it('should support multiple levels of inline partials', function() {
     var string = '{{#> layout}}{{#*inline "subcontent"}}subcontent{{/inline}}{{/layout}}';
     var partials = {