modifyNodes can now work with promises

README.md

@@ -108,6 +108,8 @@ Output:
 <p>after</p>
 ```
 
+You can also return a promise from either function and it will work fine.
+
 #### `validateNode(node)`
 
 Given a single reshape AST node, checks it for formatting errors. Example:

lib/index.js

@@ -1,30 +1,43 @@
+const when = require('when')
 const UtilError = require('./error')
 
 exports.modifyNodes = function modifyNodes (tree, criteria, transform) {
-  return tree.reduce((m, node) => {
-    // bottom-up recurse
-    if (node.type === 'tag' && node.content) {
-      node.content = modifyNodes(node.content, criteria, transform)
-    }
-
-    // if it doesn't match the criteria, move on
-    if (!criteria(node)) { m.push(node); return m }
+  return when.reduce(tree, (m, node) => {
+    // this will resolve immediately unless the node has children and needs to
+    // recurse, in which case it will wait for the recursion to finish before
+    // resolving
+    let maybeRecurse = when.resolve()
 
-    // if it does, run the user transform
-    const output = transform(node)
-
-    // push the output into the tree if it's a valid type
-    if (Array.isArray(output)) {
-      m = m.concat(output)
-    } else if (typeof output === 'object') {
-      m.push(output)
-    } else if (!output) {
-      // no node added
-    } else {
-      throw new UtilError('invalid replacement node', output)
+    // bottom-up recurse if there is a tag with contents
+    if (node.type === 'tag' && node.content) {
+      maybeRecurse = modifyNodes(node.content, criteria, transform)
+        .tap((content) => { node.content = content })
     }
 
-    return m
+    // after the recurse has finished if applicable, test the node for the user
+    // criteria and modify if applicable
+    return maybeRecurse.then(() => {
+      // run the criteria function (can be a promise)
+      return when.resolve(criteria(node)).then((processNode) => {
+        // if it doesn't match the criteria, move on
+        if (!processNode) { m.push(node); return m }
+
+        // if it does, run the user transform (can be a promise)
+        return when.resolve(transform(node)).then((output) => {
+          // push the output into the tree if it's a valid type
+          if (Array.isArray(output)) {
+            m.push(...output)
+          } else if (typeof output === 'object') {
+            m.push(output)
+          } else if (!output) {
+            // no node added
+          } else {
+            throw new UtilError('invalid replacement node', output)
+          }
+          return m
+        })
+      })
+    })
   }, [])
 }
 

package.json

@@ -27,5 +27,8 @@
     "coveralls": "nyc report --reporter=text-lcov | coveralls",
     "pretest": "standard | snazzy",
     "test": "nyc ava"
+  },
+  "dependencies": {
+    "when": "^3.7.7"
   }
 }

test/index.js

@@ -42,6 +42,23 @@ test('modifyNodes errors when invalid return node provided', (t) => {
     .catch((err) => t.truthy(err.toString() === 'Error: invalid replacement node\nFrom: plugin-util\n\nNode: "foo"'))
 })
 
+test('modifyNodes works with promises', (t) => {
+  function plugin (tree) {
+    return util.modifyNodes(tree, (n) => {
+      return new Promise((resolve) => resolve(n.name === 'p'))
+    }, (node) => {
+      return new Promise((resolve) => {
+        node.content[0].content = 'replaced!'
+        resolve(node)
+      })
+    })
+  }
+
+  return reshape({ plugins: plugin })
+    .process(readFileSync(path.join(fixtures, 'basic.html'), 'utf8'))
+    .then((res) => t.truthy(res.output() === '<div class="wow">\n  <p>replaced!</p>\n</div>\n'))
+})
+
 test('validateNode', (t) => {
   util.validateNode({
     type: 'tag',