diff --git a/src/components/modals/TransformModal.scss b/src/components/modals/TransformModal.scss
index d551557..3a37714 100644
--- a/src/components/modals/TransformModal.scss
+++ b/src/components/modals/TransformModal.scss
@@ -1,2 +1,37 @@
 @import '../../styles.scss';
 @import './Modal.scss';
+
+.jsoneditor-modal.transform {
+
+  .description {
+    padding-bottom: $padding;
+  }
+
+  label {
+    font-weight: bold;
+    padding: $padding/2 0;
+    display: block;
+  }
+
+  textarea.query,
+  textarea.preview {
+    border: 1px solid $border-gray;
+    border-radius: $border-radius;
+    outline: none;
+    width: 100%;
+    height: 100px;
+    resize: vertical; // prevent resizing horizontally
+    box-sizing: border-box;
+    padding: $padding / 2;
+    font-family: $font-family-mono;
+    font-size: $font-size-mono;
+  }
+
+  textarea.preview {
+    height: 200px;
+    
+    &.error {
+      color: $red;
+    }
+  }
+}
diff --git a/src/components/modals/TransformModal.svelte b/src/components/modals/TransformModal.svelte
index 70ba808..2b05055 100644
--- a/src/components/modals/TransformModal.svelte
+++ b/src/components/modals/TransformModal.svelte
@@ -2,15 +2,94 @@
 
 <script>
   import { getContext } from 'svelte'
+import { compileJSONPointer } from '../../utils/jsonPointer';
   import Header from './Header.svelte'
 
+  export let id
+  export let json
+  export let rootPath
+  export let onTransform
+
   const {close} = getContext('simple-modal')
+
+  let query = 'function query (data) {\n  return data\n}'
+  let previewHasError = false
+  let preview = ''
+
+  function evalTransform(json, query) {
+    // TODO: replace unsafe eval with a JS based query language 
+    const queryFn = eval(`(${query})`)
+    return queryFn(json)
+  }
+
+  function previewTransform(json, query) {
+    try {
+      const jsonTransformed = evalTransform(json, query)
+
+      preview = JSON.stringify(jsonTransformed, null, 2) // TODO: limit preview length
+      previewHasError = false
+    } catch (err) {
+      preview = err.toString()
+      previewHasError = true
+    }
+  }
+
+  $: {
+    previewTransform(json, query)
+  }
+
+  function handleTransform () {
+    try {
+      const jsonTransformed = evalTransform(json, query)
+
+      onTransform([
+        {
+          op: 'replace',
+          path: compileJSONPointer(rootPath),
+          value: jsonTransformed
+        }
+      ])
+      
+      close()
+    } catch (err) {
+      // this should never occur since we can only press the Transform 
+      // button when creating a preview was succesful
+      console.error(err)
+      preview = err.toString()
+      previewHasError = true
+    }
+
+  }
+
 </script>
 
 <div class="jsoneditor-modal transform">
   <Header title='Transform' />
   <div class="contents">
-    TODO...
+    <div class='description'>
+      Enter a JavaScript function to filter, sort, or transform the data.
+    </div>
+
+    <label>Query</label>
+    <textarea class="query" bind:value={query} />
+
+    <label>Preview</label>
+    <textarea
+      class="preview" 
+      class:error={previewHasError}
+      bind:value={preview} 
+      readonly 
+    />
+
+    <div class="actions">
+      <button 
+        class="primary"
+        on:click={handleTransform} 
+        disabled={previewHasError}
+      >
+        Transform
+      </button>
+    </div>
   </div>
 </div>
 
diff --git a/src/components/treemode/TreeMode.svelte b/src/components/treemode/TreeMode.svelte
index 899dc7b..5cae70a 100644
--- a/src/components/treemode/TreeMode.svelte
+++ b/src/components/treemode/TreeMode.svelte
@@ -19,7 +19,8 @@
   import {
     createPathsMap,
     createSelectionFromOperations,
-    expandSelection
+    expandSelection,
+findRootPath
   } from '../../logic/selection.js'
   import { isContentEditableDiv } from '../../utils/domUtils.js'
   import {
@@ -43,6 +44,7 @@
 
   const { open } = getContext('simple-modal')
   const sortModalId = uniqueId()
+  const transformModalId = uniqueId()
 
   let divContents
   let domHiddenInput
@@ -264,11 +266,7 @@
   }
 
   function handleSort () {
-    const rootPath = selection && selection.paths
-      ? selection.paths.length > 1 
-        ? initial(first(selection.paths)) // the parent path of the paths
-        : first(selection.paths) // the first and only path
-      : []
+    const rootPath = findRootPath(selection)
 
     open(SortModal, {
       id: sortModalId,
@@ -288,7 +286,26 @@
   }
 
   function handleTransform () {
-    open(TransformModal, {}, SIMPLE_MODAL_OPTIONS)
+    const rootPath = findRootPath(selection)
+
+    open(TransformModal, {
+      id: transformModalId,
+      json: getIn(doc, rootPath),
+      rootPath,
+      onTransform: async (operations) => {
+        console.log('onTransform', rootPath, operations)
+        patch(operations, selection)
+
+        // FIXME: replaced node should keep it's expanded state (if expanded)
+        
+      }
+    }, {
+      ...SIMPLE_MODAL_OPTIONS, 
+      styleWindow: {
+        ...SIMPLE_MODAL_OPTIONS.styleWindow,
+        width: '600px'
+      }
+    })
   }
 
   async function handleSearchText (text) {
diff --git a/src/logic/selection.js b/src/logic/selection.js
index b87f749..21b8f62 100644
--- a/src/logic/selection.js
+++ b/src/logic/selection.js
@@ -17,14 +17,14 @@ import { isObject } from '../utils/typeUtils.js'
 export function expandSelection (doc, state, anchorPath, focusPath) {
   if (isEqual(anchorPath, focusPath)) {
     // just a single node
-    return [ anchorPath ]
+    return [anchorPath]
   } else {
     // multiple nodes
-    let sharedPath = findSharedPath(anchorPath, focusPath)
+    const sharedPath = findSharedPath(anchorPath, focusPath)
 
     if (anchorPath.length === sharedPath.length || focusPath.length === sharedPath.length) {
       // a parent and a child, like ['arr', 1] and ['arr']
-      return [ sharedPath ]
+      return [sharedPath]
     }
 
     const anchorKey = anchorPath[sharedPath.length]
@@ -66,8 +66,7 @@ export function expandSelection (doc, state, anchorPath, focusPath) {
 }
 
 /**
- * 
- * @param {Selection} selection 
+ * @param {Selection} selection
  * @return {Path} Returns parent path
  */
 export function getParentPath (selection) {
@@ -94,7 +93,7 @@ export function createSelectionFromOperations (operations) {
     .filter(operation => operation.op === 'add' || operation.op === 'copy')
     .map(operation => parseJSONPointer(operation.path))
 
-  return  {
+  return {
     paths,
     pathsMap: createPathsMap(paths)
   }
@@ -123,10 +122,21 @@ export function createPathsMap (paths) {
  */
 // TODO: write unit tests for findSharedPath
 export function findSharedPath (path1, path2) {
-  let i = 0;
+  let i = 0
   while (i < path1.length && path1[i] === path2[i]) {
-    i++;
+    i++
   }
 
   return path1.slice(0, i)
 }
+
+/**
+ * @param {Selection} selection
+ */
+export function findRootPath (selection) {
+  return selection && selection.paths
+    ? selection.paths.length > 1
+      ? initial(first(selection.paths)) // the parent path of the paths
+      : first(selection.paths) // the first and only path
+    : []
+}
diff --git a/src/logic/selection.test.js b/src/logic/selection.test.js
index 0197d06..85e947b 100644
--- a/src/logic/selection.test.js
+++ b/src/logic/selection.test.js
@@ -1,15 +1,15 @@
 import assert from 'assert'
-import { expandSelection, getParentPath } from './selection.js'
+import { expandSelection, getParentPath, findRootPath } from './selection.js'
 import { syncState } from './documentState.js'
 
-describe ('selection', () => {
+describe('selection', () => {
   const doc = {
-    "obj": {
-      "arr": [1,2, {"first":3,"last":4}]
+    obj: {
+      arr: [1, 2, { first: 3, last: 4 }]
     },
-    "str": "hello world",
-    "nill": null,
-    "bool": false
+    str: 'hello world',
+    nill: null,
+    bool: false
   }
   const state = syncState(doc, undefined, [], () => true)
 
@@ -67,12 +67,27 @@ describe ('selection', () => {
   })
 
   it('should get parent path from a selection', () => {
-      assert.deepStrictEqual(getParentPath({ beforePath: ['a', 'b']}), ['a'])
-      assert.deepStrictEqual(getParentPath({ appendPath: ['a', 'b']}), ['a', 'b'])
-      assert.deepStrictEqual(getParentPath({ paths:[
+    assert.deepStrictEqual(getParentPath({ beforePath: ['a', 'b'] }), ['a'])
+    assert.deepStrictEqual(getParentPath({ appendPath: ['a', 'b'] }), ['a', 'b'])
+    assert.deepStrictEqual(getParentPath({
+      paths: [
         ['a', 'b'],
         ['a', 'c'],
         ['a', 'd']
-      ]}), ['a'])
+      ]
+    }), ['a'])
+  })
+
+  it('should find the root path from a selection', () => {
+    assert.deepStrictEqual(findRootPath({
+      paths: [
+        ['a', 'b'],
+        ['a', 'c'],
+        ['a', 'd']
+      ]
+    }), ['a'])
+    assert.deepStrictEqual(findRootPath({ beforePath: ['a', 'b'] }), [])
+    assert.deepStrictEqual(findRootPath({ appendPath: ['a', 'b'] }), [])
+    assert.deepStrictEqual(findRootPath(), [])
   })
 })