Symbols must be stringified explicitly

Apparently (nodejs/node#927 (comment)),
Symbols cannot be implicitly coerced to string.

Template stringification leverages implicit coercion, so we must
stringify propKey before sending it to the template.

Options for explicitly stringifying:

- `String(propKey)`
- `String.prototype.toString(propKey)`

Differences:

```
> String(undefined)
'undefined'
> String.prototype.toString(undefined)
''
```

In this case, `propKey` represents the key for the stubbed function. If
the key is `undefined`, it would be implicitly coerced to `"undefined"`
as the property key. So `String()` is the explicit converstion we want.

Four instances of this issue, each where tdFunction is invoked with a
dynamic name.

regression/src/constructor-test.coffee

@@ -72,6 +72,17 @@ describe 'td.constructor', ->
     And -> @fakeInstance.foo.toString() == '[test double for "#foo"]'
 
 
+  if (global.Symbol)
+    describe 'edge case: being given a Symbol as function name', ->
+      Given -> @symbolFoo = Symbol('foo')
+      Given -> @fakeConstructor = td.constructor([@symbolFoo])
+      Given -> @fakeInstance = new @fakeConstructor('biz')
+      Then -> @fakeConstructor.prototype[@symbolFoo] == @fakeInstance[@symbolFoo]
+      And -> td.verify(new @fakeConstructor('biz'))
+      And -> td.explain(@fakeInstance[@symbolFoo]).isTestDouble == true
+      And -> @fakeConstructor.toString() == '[test double for "(unnamed constructor)"]'
+      And -> @fakeInstance.toString() == '[test double instance of constructor]'
+      And -> @fakeInstance[@symbolFoo].toString() == '[test double for "#Symbol(foo)"]'
 
   describe 'edge case: being given a function without prototypal methods', ->
     Given -> @boringFunc = ->

regression/src/object-test.coffee

@@ -25,6 +25,15 @@ describe 'td.object', ->
     And -> @testDouble.toString() == '[test double object]'
     And -> @testDouble.bam.toString() == '[test double for ".bam"]'
 
+  if (global.Symbol)
+    context 'making a test double based on a Symbol', ->
+      Given -> @symbolFoo = Symbol('foo')
+      Given -> @testDouble = td.object([@symbolFoo])
+      When -> td.when(@testDouble[@symbolFoo]()).thenReturn('zing!')
+      Then -> @testDouble[@symbolFoo]() == 'zing!'
+      And -> @testDouble.toString() == '[test double object]'
+      And -> @testDouble[@symbolFoo].toString() == '[test double for ".Symbol(foo)"]'
+
   describe 'passing a function to td.object erroneously (1.x)', ->
     When -> try td.object(->) catch e then @result = e
     Then -> expect(@result.message).to.contain(
@@ -52,6 +61,11 @@ describe 'td.object', ->
         Given -> @testDouble = td.object()
         Then -> @testDouble.toString() == '[test double object]'
         Then -> @testDouble.lol.toString() == '[test double for ".lol"]'
+
+      if (global.Symbol)
+        context 'with Symbol propKey', ->
+          And -> @testDouble[Symbol('foo')].toString() == '[test double for "thing.Symbol(foo)"]'
+
   else
     describe 'getting an error message', ->
       When -> try

src/constructor.js

@@ -13,7 +13,7 @@ var fakeConstructorFromNames = (funcNames) => {
       '[test double instance of constructor]'
 
     _.each(funcNames, (funcName) => {
-      fakeConstructor.prototype[funcName] = tdFunction(`#${funcName}`)
+      fakeConstructor.prototype[funcName] = tdFunction(`#${String(funcName)}`)
     })
   })
 }

src/imitate/create-imitation.js

@@ -11,7 +11,7 @@ export default (original, names) => {
       return original
     } else {
       // TODO: this will become src/function/create and include parent reference instead of name joining here
-      return tdFunction(names.join('') || '(anonymous function)')
+      return tdFunction(names.map(String).join('') || '(anonymous function)')
     }
   } else {
     return _.clone(original)

src/object.js

@@ -26,7 +26,7 @@ var fakeObject = (nameOrType, config) => {
 
 var createTestDoublesForFunctionNames = (names) =>
   _.transform(names, (acc, funcName) => {
-    acc[funcName] = tdFunction(`.${funcName}`)
+    acc[funcName] = tdFunction(`.${String(funcName)}`)
   })
 
 var createTestDoubleViaProxy = (name, config) => {
@@ -35,7 +35,7 @@ var createTestDoubleViaProxy = (name, config) => {
   return new Proxy(obj, {
     get (target, propKey, receiver) {
       if (!obj.hasOwnProperty(propKey) && !_.includes(config.excludeMethods, propKey)) {
-        obj[propKey] = tdFunction(`${nameOf(name)}.${propKey}`)
+        obj[propKey] = tdFunction(`${nameOf(name)}.${String(propKey)}`)
       }
       return obj[propKey]
     }

test/unit/imitate/create-imitation.test.js

@@ -30,6 +30,20 @@ module.exports = {
 
     assert.deepEqual(result, 'fake thing')
   },
+  'a function with symbols' () {
+    if (!global.Symbol) return
+
+    const someFunc = () => {}
+    const symFoo = Symbol('foo')
+    const symBar = Symbol('bar')
+    td.when(tdFunction('Symbol(foo)Symbol(bar)'))
+      .thenReturn('fake thing')
+    td.when(isGenerator(someFunc)).thenReturn(false)
+
+    const result = subject(someFunc, [symFoo, symBar])
+
+    assert.deepEqual(result, 'fake thing')
+  },
   'other instances': () => {
     const original = {a: 'b'}