Add support for passing a child

Author: wooorm

index.js

@@ -89,6 +89,14 @@ function isLiteral(parent, index) {
     throw new Error('Parent must be a node')
   }
 
+  if (index !== null && typeof index === 'object' && 'type' in index) {
+    index = parent.children.indexOf(index)
+
+    if (index === -1) {
+      throw new Error('Node must be a child of `parent`')
+    }
+  }
+
   if (isNaN(index)) {
     throw new Error('Index must be a number')
   }

readme.md

@@ -75,10 +75,10 @@ quux
 
 ## API
 
-### `isLiteral(parent, index)`
+### `isLiteral(parent, index|child)`
 
-Check if the node in `parent` at `position` is enclosed
-by matching delimiters.
+Check if the `child` in `parent` is enclosed by matching delimiters.
+If `index` is given, the [child][] of `parent` at that [index][] is checked.
 
 For example, `foo` is literal in the following samples:
 
@@ -141,3 +141,7 @@ abide by its terms.
 [coc]: https://github.com/syntax-tree/.github/blob/master/code-of-conduct.md
 
 [nlcst]: https://github.com/syntax-tree/nlcst
+
+[index]: https://github.com/syntax-tree/unist#index
+
+[child]: https://github.com/syntax-tree/unist#child

test.js

@@ -28,9 +28,22 @@ test('isLiteral()', function(t) {
       isLiteral({children: []})
     },
     /Index must be a number/,
-    'should throw without index'
+    'should throw without node'
   )
 
+  t.throws(
+    function() {
+      isLiteral({children: []}, {type: 'a'})
+    },
+    /Node must be a child of `parent`/,
+    'should throw if `node` is not in `parent`'
+  )
+
+  t.doesNotThrow(function() {
+    var n = {type: 'a'}
+    isLiteral({children: [n]}, n)
+  }, 'should not throw if `node` is in `parent`')
+
   t.doesNotThrow(function() {
     process('Well? Ha! Funky', function(node, index, parent) {
       assert.strictEqual(isLiteral(parent, index), false)