Add error on misconfigured fence

Author: wooorm

lib/handle/code.js

@@ -3,11 +3,12 @@ module.exports = code
 var repeat = require('repeat-string')
 var streak = require('longest-streak')
 var formatCodeAsIndented = require('../util/format-code-as-indented')
+var checkFence = require('../util/check-fence')
 var indentLines = require('../util/indent-lines')
 var safe = require('../util/safe')
 
 function code(node, _, context) {
-  var marker = context.options.fence || '`'
+  var marker = checkFence(context)
   var raw = node.value || ''
   var suffix = marker === '`' ? 'GraveAccent' : 'Tilde'
   var value

lib/util/check-fence.js

@@ -0,0 +1,15 @@
+module.exports = checkFence
+
+function checkFence(context) {
+  var marker = context.options.fence || '`'
+
+  if (marker !== '`' && marker !== '~') {
+    throw new Error(
+      'Cannot serialize code with `' +
+        marker +
+        '` for `options.fence`, expected `` ` `` or `~`'
+    )
+  }
+
+  return marker
+}

test.js

@@ -509,6 +509,14 @@ test('break', function (t) {
 test('code (flow)', function (t) {
   t.equal(to({type: 'code'}), '```\n```', 'should support empty code')
 
+  t.throws(
+    function () {
+      to({type: 'code'}, {fence: '+'})
+    },
+    /Cannot serialize code with `\+` for `options\.fence`, expected `` ` `` or `~`/,
+    'should throw on when given an incorrect `fence`'
+  )
+
   t.equal(
     to({type: 'code', value: 'a'}),
     '    a',