Throw an error if an invalid test is passed in

Author: wooorm

index.js

@@ -22,10 +22,17 @@ function headingRange(node, options, callback) {
   }
 
   /* Regex */
-  if ('exec' in test) {
+  if (test && 'exec' in test) {
     test = wrapExpression(test);
   }
 
+  if (typeof test !== 'function') {
+    throw new Error(
+      'Expected `string`, `regexp`, or `function` for `test`, ' +
+      'not `' + test + '`'
+    );
+  }
+
   search(node, test, ignoreFinalDefinitions, callback);
 }
 

package.json

@@ -44,6 +44,9 @@
   "xo": {
     "space": true,
     "esnext": false,
+    "rules": {
+      "unicorn/prefer-type-error": "off"
+    },
     "ignore": [
       "mdast-util-heading-range.js"
     ]

test.js

@@ -5,10 +5,26 @@ var remark = require('remark');
 var heading = require('./');
 
 test('mdast-util-heading-range()', function (t) {
-  t.plan(55);
+  t.plan(57);
 
   t.equal(typeof heading, 'function', 'should be a function');
 
+  t.throws(
+    function () {
+      heading({type: 'root', chilren: []}, null, function () {});
+    },
+    /^Error: Expected `string`, `regexp`, or `function` for `test`, not `null`$/,
+    'should throw when `null` is passed in'
+  );
+
+  t.throws(
+    function () {
+      heading({type: 'root', chilren: []}, undefined, function () {});
+    },
+    /^Error: Expected `string`, `regexp`, or `function` for `test`, not `undefined`$/,
+    'should throw when `undefined` is passed in'
+  );
+
   t.equal(
     process(t, [
       '# Fo',