fix: add guard to if & unless helpers

fixes #1548

- add a guard to show readable syntax error for if / unless helper
- prevents against 3 different errors that can be generated by different systax errors

lib/handlebars/helpers/if.js

@@ -1,7 +1,9 @@
-import {isEmpty, isFunction} from '../utils';
+import { isEmpty, isFunction } from '../utils';
+import Exception from '../exception';
 
 export default function(instance) {
   instance.registerHelper('if', function(conditional, options) {
+    if (arguments.length != 2) { throw new Exception('#if requires exactly one argument');}
     if (isFunction(conditional)) { conditional = conditional.call(this); }
 
     // Default behavior is to render the positive path if the value is truthy and not empty.
@@ -15,6 +17,7 @@ export default function(instance) {
   });
 
   instance.registerHelper('unless', function(conditional, options) {
+    if (arguments.length != 2) { throw new Exception('#unless requires exactly one argument');}
     return instance.helpers['if'].call(this, conditional, {fn: options.inverse, inverse: options.fn, hash: options.hash});
   });
 }

lib/handlebars/helpers/with.js

@@ -1,7 +1,9 @@
-import {appendContextPath, blockParams, createFrame, isEmpty, isFunction} from '../utils';
+import { appendContextPath, blockParams, createFrame, isEmpty, isFunction } from '../utils';
+import Exception from '../exception';
 
 export default function(instance) {
   instance.registerHelper('with', function(context, options) {
+    if (arguments.length != 2) { throw new Exception('#with requires exactly one argument');}
     if (isFunction(context)) { context = context.call(this); }
 
     let fn = options.fn;

spec/helpers.js

@@ -767,69 +767,69 @@ describe('helpers', function() {
     });
   });
 
-  describe.only('built-in helpers malformed arguments ', function() {
+  describe('built-in helpers malformed arguments ', function() {
     it('if helper - too few arguments', function() {
       var template = CompilerContext.compile('{{#if}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'hash' of undefined/);
+        }, undefined, /#if requires exactly one argument/);
     });
 
     it('if helper - too many arguments, string', function() {
       var template = CompilerContext.compile('{{#if test "string"}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'includeZero' of undefined/);
+        }, undefined, /#if requires exactly one argument/);
     });
 
     it('if helper - too many arguments, undefined', function() {
       var template = CompilerContext.compile('{{#if test undefined}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'hash' of undefined/);
+        }, undefined, /#if requires exactly one argument/);
     });
 
     it('if helper - too many arguments, null', function() {
       var template = CompilerContext.compile('{{#if test null}}{{/if}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'hash' of null/);
+        }, undefined, /#if requires exactly one argument/);
     });
 
     it('unless helper - too few arguments', function() {
       var template = CompilerContext.compile('{{#unless}}{{/unless}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'inverse' of undefined/);
+        }, undefined, /#unless requires exactly one argument/);
     });
 
     it('unless helper - too many arguments', function() {
       var template = CompilerContext.compile('{{#unless test null}}{{/unless}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'inverse' of null/);
+        }, undefined, /#unless requires exactly one argument/);
     });
 
     it('with helper - too few arguments', function() {
       var template = CompilerContext.compile('{{#with}}{{/with}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /Cannot read property 'fn' of undefined/);
+        }, undefined, /#with requires exactly one argument/);
     });
 
     it('with helper - too many arguments', function() {
       var template = CompilerContext.compile('{{#with test "string"}}{{/with}}');
         shouldThrow(function() {
 
             template({});
-        }, undefined, /options.inverse is not a function/);
+        }, undefined, /#with requires exactly one argument/);
     });
   });
 });