Fix exception for non-string identifiers

Related-to gatsbyjs/gatsby#16578.
Closes GH-34.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Co-authored-by: Jason Lengstorf <jason@lengstorf.com>
Co-authored-by: Titus Wormer <tituswormer@gmail.com>

Author: jlengstorf

lib/handlers/footnote-reference.js

@@ -6,7 +6,7 @@ var u = require('unist-builder')
 
 function footnoteReference(h, node) {
   var footnoteOrder = h.footnoteOrder
-  var identifier = node.identifier
+  var identifier = String(node.identifier)
 
   if (footnoteOrder.indexOf(identifier) === -1) {
     footnoteOrder.push(identifier)

lib/index.js

@@ -85,7 +85,7 @@ function factory(tree, options) {
   }
 
   function onfootnotedefinition(definition) {
-    var id = definition.identifier.toUpperCase()
+    var id = String(definition.identifier).toUpperCase()
 
     // Mimick CM behavior of link definitions.
     // See: <https://github.com/syntax-tree/mdast-util-definitions/blob/8d48e57/index.js#L26>.

test/footnote-reference.js

@@ -35,5 +35,20 @@ test('FootnoteReference', function(t) {
     'should render `footnoteReference`s (#2)'
   )
 
+  t.deepEqual(
+    to(u('footnoteReference', {identifier: 1})),
+    u('element', {tagName: 'sup', properties: {id: 'fnref-1'}}, [
+      u(
+        'element',
+        {
+          tagName: 'a',
+          properties: {href: '#fn-1', className: ['footnote-ref']}
+        },
+        [u('text', '1')]
+      )
+    ]),
+    'should not fail on non-string identifiers'
+  )
+
   t.end()
 })