Restrict implicit export of final mapping expression (fix #91)

BREAKING CHANGE: In .js/.coffee mapping files, for the final expression
to be implicitly exported, it must be an object expression (literal) or
function expression.

The idea is that such an expression wouldn't do much by itself.
(In particular, it cannot be an assignment or function call.
We were getting too many false positives in this case.)

README.md

@@ -172,7 +172,9 @@ The code specifies a `mapping` in one of three ways:
 
 1. `export default mapping` (ECMAScript modules style)
 2. `exports.default = mapping` (CommonJS modules style)
-3. Writing a `mapping` expression at the end of the file (implicit export).
+3. Writing a `mapping` expression at the end of the file (implicit export),
+   which must be a top-level object or function expression
+   (without e.g. being assigned to a variable).
 
 In any case, `mapping` should be one of the following types of **mapping**
 objects:

examples/chess/graph.coffee

@@ -72,5 +72,3 @@ svgtiler.afterRender (render) ->
           />
 
   <svg>{edges}</svg>
-
-null

examples/mario/mario.coffee

@@ -41,7 +41,7 @@ map =
   spiny_left: <symbol viewBox="0 0 16 16" z-index="1"><image y="1" xlink:href="spiny_left.png"/></symbol>
   spiny_right: <symbol viewBox="0 0 16 16" z-index="1"><image y="1" xlink:href="spiny_right.png"/></symbol>
 
-build = (key) ->
+export default build = (key) ->
   ### Keys:
   "a,b" expands to the equivalent of a beneath b
   "a+x+y" expands to a shifted by (x, y)

examples/mario/palette_castle.js

@@ -1,2 +1 @@
 share.palette = 'castle'
-null

examples/mario/palette_overworld.js

@@ -1,2 +1 @@
 share.palette = 'overworld'
-null

examples/mario/palette_underground.js

@@ -1,2 +1 @@
 share.palette = 'underground'
-null

examples/mario/palette_underwater.js

@@ -1,2 +1 @@
 share.palette = 'underwater'
-null

src/svgtiler.coffee

@@ -35,11 +35,16 @@ unless window?
           if types.isExportDefaultDeclaration part
             return  # already an export default, so don't add one
         last = body[body.length-1]
-        if last.node.type == 'ExpressionStatement'
-          exportLast = types.exportDefaultDeclaration last.node.expression
-          exportLast.leadingComments = last.node.leadingComments
-          exportLast.innerComments = last.node.innerComments
-          exportLast.trailingComments = last.node.trailingComments
+        lastNode = last.node
+        if types.isExpressionStatement(last) and (
+          types.isObjectExpression(lastNode.expression) or
+          types.isFunctionExpression(lastNode.expression)
+          # not AssignmentExpression
+        )
+          exportLast = types.exportDefaultDeclaration lastNode.expression
+          exportLast.leadingComments = lastNode.leadingComments
+          exportLast.innerComments = lastNode.innerComments
+          exportLast.trailingComments = lastNode.trailingComments
           last.replaceWith exportLast
         undefined