Improve error message with invalid td.object call

Fixes #224

Author: searls

src/object.js

@@ -17,10 +17,12 @@ var fakeObject = (nameOrType, config) => {
     return createTestDoublesForPlainObject(nameOrType)
   } else if (_.isArray(nameOrType)) {
     return createTestDoublesForFunctionNames(nameOrType)
+  } else if (_.isString(nameOrType) || nameOrType === undefined) {
+    return createTestDoubleViaProxy(nameOrType, withDefaults(config))
   } else if (_.isFunction(nameOrType)) {
-    return ensureFunctionIsNotPassed()
+    ensureFunctionIsNotPassed()
   } else {
-    return createTestDoubleViaProxy(nameOrType, withDefaults(config))
+    ensureOtherGarbageIsNotPassed()
   }
 }
 
@@ -63,6 +65,20 @@ Did you mean \`td.object(['${name}'])\`?\
   }
 }
 
+var ensureFunctionIsNotPassed = () =>
+  log.error('td.object', `Functions are not valid arguments to \`td.object\` (as of [email protected]). Please use \`td.function()\` or \`td.constructor()\` instead for creating fake functions.`)
+
+var ensureOtherGarbageIsNotPassed = () =>
+    log.error('td.object', `\
+To create a fake object with td.object(), pass it a plain object that contains
+functions, an array of function names, or (if your runtime supports ES Proxy
+objects) a string name.
+
+If you passed td.object an instance of a custom type, consider passing the
+type's constructor to \`td.constructor()\` instead.
+`)
+
+
 var withDefaults = (config) =>
   _.extend({}, DEFAULT_OPTIONS, config)
 
@@ -75,6 +91,3 @@ var nameOf = (nameOrType) =>
   _.isString(nameOrType)
     ? nameOrType
     : ''
-
-var ensureFunctionIsNotPassed = () =>
-  log.error('td.object', `Functions are not valid arguments to \`td.object\` (as of [email protected]). Please use \`td.function()\` or \`td.constructor()\` instead for creating fake functions.`)

test/src/object-test.coffee

@@ -12,6 +12,12 @@ describe 'td.object', ->
     And -> @testDouble.now.toString() == '[test double for ".now"]'
     And -> @testDouble.otherThing == 8
 
+  describe.only 'creating an object that is an instance of a prototypal thing', ->
+    Given -> @type = class Thing
+      foo: -> 'bar'
+    When -> try td.object(new @type()) catch e then @error = e
+    Then -> expect(@error.message).to.contain('To create a fake object')
+
   describe 'making a test double based on an array of strings', ->
     Given -> @testDouble = td.object(['biz','bam','boo'])
     When -> td.when(@testDouble.biz()).thenReturn('zing!')