diff --git a/CHANGELOG.md b/CHANGELOG.md
index f9a93687..a3b9c635 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+v0.6.42
+* Added "+ add field" button to web ui inspector in attribute editing mode.
+
+
 v0.6.41
 * Bug fixes
 
diff --git a/package-lock.json b/package-lock.json
index 0de1367d..0c8789e9 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "mapshaper",
-  "version": "0.6.41",
+  "version": "0.6.42",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "mapshaper",
-      "version": "0.6.41",
+      "version": "0.6.42",
       "license": "MPL-2.0",
       "dependencies": {
         "@placemarkio/tokml": "^0.3.3",
diff --git a/package.json b/package.json
index 43c41cfa..deb9718f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "mapshaper",
-  "version": "0.6.41",
+  "version": "0.6.42",
   "description": "A tool for editing vector datasets for mapping and GIS.",
   "keywords": [
     "shapefile",
diff --git a/src/cli/mapshaper-command-utils.mjs b/src/cli/mapshaper-command-utils.mjs
index 83209929..2bc97fd0 100644
--- a/src/cli/mapshaper-command-utils.mjs
+++ b/src/cli/mapshaper-command-utils.mjs
@@ -17,6 +17,16 @@ export function applyCommandToEachLayer(func, targetLayers) {
   }, []);
 }
 
+export function applyCommandToEachTarget(func, targets) {
+  var args = utils.toArray(arguments).slice(2);
+  targets.forEach(function(target) {
+    var result = func.apply(null, [target].concat(args));
+    if (result) {
+      error('Unexpected output from command');
+    }
+  });
+}
+
 function isLayer(arg) {
   return arg && (Array.isArray(arg.shapes) || !!arg.data || !!arg.geometry_type);
 }
diff --git a/src/cli/mapshaper-run-command.mjs b/src/cli/mapshaper-run-command.mjs
index e220fbc6..d7b6b986 100644
--- a/src/cli/mapshaper-run-command.mjs
+++ b/src/cli/mapshaper-run-command.mjs
@@ -13,7 +13,7 @@ import { stop, error, UserError, verbose } from '../utils/mapshaper-logging';
 import utils from '../utils/mapshaper-utils';
 import cmd from '../mapshaper-cmd';
 import { stashVar, clearStash } from '../mapshaper-stash';
-import { applyCommandToEachLayer } from '../cli/mapshaper-command-utils';
+import { applyCommandToEachLayer, applyCommandToEachTarget } from '../cli/mapshaper-command-utils';
 
 import '../commands/mapshaper-add-shape';
 import '../commands/mapshaper-affine';
@@ -92,7 +92,7 @@ import '../commands/mapshaper-subdivide';
 function commandAcceptsMultipleTargetDatasets(name) {
   return name == 'rotate' || name == 'info' || name == 'proj' || name == 'require' ||
     name == 'drop' || name == 'target' || name == 'if' || name == 'elif' ||
-    name == 'else' || name == 'endif' || name == 'run' || name == 'i';
+    name == 'else' || name == 'endif' || name == 'run' || name == 'i' || name == 'snap';
 }
 
 function commandAcceptsEmptyTarget(name) {
@@ -422,7 +422,8 @@ export async function runCommand(command, job) {
       outputLayers = cmd.sliceLayers(targetLayers, source, targetDataset, opts);
 
     } else if (name == 'snap') {
-      cmd.snap(targetDataset, opts);
+      // cmd.snap(targetDataset, opts);
+      applyCommandToEachTarget(targets, opts);
 
     } else if (name == 'sort') {
       applyCommandToEachLayer(cmd.sortFeatures, targetLayers, arcs, opts);
diff --git a/src/commands/mapshaper-snap.mjs b/src/commands/mapshaper-snap.mjs
index 9dc77cf6..fad15286 100644
--- a/src/commands/mapshaper-snap.mjs
+++ b/src/commands/mapshaper-snap.mjs
@@ -7,9 +7,10 @@ import { stop, message } from '../utils/mapshaper-logging';
 import cmd from '../mapshaper-cmd';
 import utils from '../utils/mapshaper-utils';
 
-cmd.snap = function(dataset, opts) {
+cmd.snap = function(target, opts) {
   var interval = 0;
   var snapCount = 0;
+  var dataset = target.dataset;
   var arcs = dataset.arcs;
   var arcBounds = arcs && arcs.getBounds();
   if (!arcBounds || !arcBounds.hasBounds()) {
diff --git a/src/gui/gui-alert.mjs b/src/gui/gui-alert.mjs
index b9d9242f..0a757ec4 100644
--- a/src/gui/gui-alert.mjs
+++ b/src/gui/gui-alert.mjs
@@ -4,21 +4,26 @@ export function showPopupAlert(msg, title) {
   var self = {}, html = '';
   var _cancel, _close;
   var warningRxp = /^Warning: /;
-  var el = El('div').appendTo('body').addClass('error-wrapper');
-  var infoBox = El('div').appendTo(el).addClass('error-box info-box selectable');
+  var el = El('div').appendTo('body').addClass('alert-wrapper');
+  var infoBox = El('div').appendTo(el).addClass('alert-box info-box selectable');
+  El('div').appendTo(infoBox).addClass('close2-btn').on('click', function() {
+    if (_cancel) _cancel();
+    self.close();
+  });
+  var container = El('div').appendTo(infoBox);
   if (!title && warningRxp.test(msg)) {
     title = 'Warning';
     msg = msg.replace(warningRxp, '');
   }
   if (title) {
-    html += `<div class="error-title">${title}</div>`;
+    El('div').addClass('alert-title').text(title).appendTo(container);
   }
-  html += `<p class="error-message">${msg}</p>`;
-  El('div').appendTo(infoBox).addClass('close2-btn').on('click', function() {
-    if (_cancel) _cancel();
-    self.close();
-  });
-  El('div').appendTo(infoBox).addClass('error-content').html(html);
+  var content = El('div').appendTo(infoBox);
+  if (msg) {
+    content.html(`<p class="error-message">${msg}</p>`);
+  }
+
+  self.container = function() { return content; };
 
   self.onCancel = function(cb) {
     _cancel = cb;
diff --git a/src/gui/gui-popup.mjs b/src/gui/gui-popup.mjs
index 583313fd..6a1ce4ad 100644
--- a/src/gui/gui-popup.mjs
+++ b/src/gui/gui-popup.mjs
@@ -3,6 +3,7 @@ import { utils, internal } from './gui-core';
 import { El } from './gui-el';
 import { GUI } from './gui-lib';
 import { ClickText2 } from './gui-elements';
+import { showPopupAlert } from './gui-alert';
 
 // toNext, toPrev: trigger functions for switching between multiple records
 export function Popup(gui, toNext, toPrev) {
@@ -35,8 +36,7 @@ export function Popup(gui, toNext, toPrev) {
     // stash a function for refreshing the current popup when data changes
     // while the popup is being displayed (e.g. while dragging a label)
     refresh = function() {
-      var rec = table && (editable ? table.getRecordAt(id) : table.getReadOnlyRecordAt(id)) || {};
-      render(content, rec, table, editable);
+      render(content, id, table, editable);
     };
     refresh();
     if (ids && ids.length > 1) {
@@ -75,7 +75,8 @@ export function Popup(gui, toNext, toPrev) {
     tab.show();
   }
 
-  function render(el, rec, table, editable) {
+  function render(el, recId, table, editable) {
+    var rec = table && (editable ? table.getRecordAt(recId) : table.getReadOnlyRecordAt(recId)) || {};
     var tableEl = El('table').addClass('selectable'),
         rows = 0;
     // self.hide(); // clean up if panel is already open
@@ -90,11 +91,6 @@ export function Popup(gui, toNext, toPrev) {
       }
     });
 
-    // add new field form
-    if (editable) {
-      renderNewRow(tableEl, rec, table);
-    }
-
     tableEl.appendTo(el);
     if (rows > 0) {
       // tableEl.appendTo(el);
@@ -119,15 +115,58 @@ export function Popup(gui, toNext, toPrev) {
     }
 
     if (editable) {
+      // render "add field" button
       var line = El('div').appendTo(el);
       El('span').addClass('save-menu-btn').appendTo(line).on('click', async function(e) {
-
-      }).text('add field');
+        // show "add field" dialog
+        renderAddFieldPopup(recId, table);
+      }).text('+ add field');
     }
   }
 
-  function renderNewRow(el, rec, table) {
+  function renderAddFieldPopup(recId, table) {
+    var popup = showPopupAlert('', 'Add field');
+    var el = popup.container();
+    el.addClass('option-menu');
+    var html = `<input type="text" class="field-name text-input" placeholder="field name"><br>
+    <input type="text" class="field-value text-input" placeholder="value"><br>
+    <div class="btn dialog-btn">Apply</div> <span class="inline-checkbox"><input type="checkbox" class="all" />assign value to all records</span>`;
+    el.html(html);
+
+    var name = el.findChild('.field-name');
+    name.node().focus();
+    var val = el.findChild('.field-value');
+    var box = el.findChild('.all');
+    var btn = el.findChild('.btn').on('click', function() {
+      var all = box.node().checked;
+      var nameStr = name.node().value.trim();
+      if (!nameStr) return;
+      if (table.fieldExists(nameStr)) {
+        name.node().value = '';
+        return;
+      }
+      var valStr = val.node().value.trim();
+      var value = parseUnknownType(valStr);
+      // table.addField(nameStr, function(d) {
+      //   // parse each time to avoid multiple references to objects
+      //   return (all || d == rec) ? parseUnknownType(valStr) : null;
+      // });
+
+      var cmdStr = `-each "d['${nameStr}'] = `;
+      if (!all) {
+        cmdStr += `this.id != ${recId} ? null : `;
+      }
+      valStr = JSON.stringify(JSON.stringify(value));
+      cmdStr = valStr.replace('"', cmdStr);
 
+      gui.console.runMapshaperCommands(cmdStr, function(err) {
+        if (!err) {
+          popup.close();
+        } else {
+          console.error(err);
+        }
+      });
+    });
   }
 
   function renderRow(table, rec, key, type, editable) {
@@ -163,12 +202,10 @@ export function Popup(gui, toNext, toPrev) {
     input.on('change', function(e) {
       var val2 = parser(input.value()),
           strval2 = formatInspectorValue(val2, type);
-      if (strval == strval2) {
-        // contents unchanged
-      } else if (val2 === null && type != 'object') { // allow null objects
+      if (val2 === null && type != 'object') { // allow null objects
         // invalid value; revert to previous value
         input.value(strval);
-      } else {
+      } else if (strval != strval2) {
         // field content has changed
         strval = strval2;
         gui.dispatchEvent('data_preupdate', {FID: currId}); // for undo/redo
@@ -233,6 +270,14 @@ var inputParsers = {
   }
 };
 
+function parseUnknownType(str) {
+  var val = inputParsers.number(str);
+  if (val !== null) return val;
+  val = inputParsers.object(str);
+  if (val !== null) return val;
+  return str;
+}
+
 function getInputParser(type) {
   return inputParsers[type || 'multiple'];
 }
diff --git a/www/index.html b/www/index.html
index 3c1a668a..1afb3425 100644
--- a/www/index.html
+++ b/www/index.html
@@ -172,7 +172,7 @@ <h4>File format</h4>
 
 		<!-- <div class="cancel-btn btn dialog-btn">Cancel</div> -->
 		<div class="save-btn btn dialog-btn">Export</div>
-		<span id="save-preference"><input type="checkbox"/>choose directory</span>
+		<span id="save-preference" class="inline-checkbox" style="display: none;"><input type="checkbox"/>choose directory</span>
 
 	</div>
 </div>
diff --git a/www/page.css b/www/page.css
index 02534cd6..ea315600 100644
--- a/www/page.css
+++ b/www/page.css
@@ -256,7 +256,7 @@ body {
 
 /* --- Error message ---------- */
 
-.error-wrapper {
+.alert-wrapper {
 	z-index: 120;
 	text-align: center;
 	position: absolute;
@@ -266,29 +266,33 @@ body {
 	left: 0;
 }
 
-.error-wrapper p.error-message {
+.alert-wrapper p.error-message {
 	margin: 1px 0 0 0;
 }
 
-.error-wrapper .dialog-btn {
-	margin-top: 8px;
-}
-
-.error-title {
+.alert-title {
 	line-height: 1.1;
 	font-weight: bold;
 	margin-bottom: 5px;
 }
 
-div.error-box {
+div.alert-title, div.error-message {
+	font-size: 16px;
+}
+
+.alert-wrapper .alert-btn {
+	margin-top: 8px;
+}
+
+div.alert-box {
 	margin-top: 42px; /* 55px; */
 	overflow: auto;
 	max-height: 70%;
 	max-width: 400px;
-	font-size: 16px;
 }
 
 
+
 /* --- Splash screen -------------- */
 
 .splash-screen #splash-screen {
@@ -513,7 +517,7 @@ body.dragover #import-options-drop-area .drop-area {
 	text-align: left;
 	margin-top: 12px;
 	margin-right: 20px;
-	padding: 12px 16px 12px 18px;
+	padding: 11px 16px 12px 18px;
 	vertical-align: top;
 	display: inline-block;
 	/* border: 1px solid #aaa; */
@@ -553,6 +557,7 @@ body.dragover #import-options-drop-area .drop-area {
 	top: 1px;
 	width: 12px;
 	height: 12px;
+	cursor: pointer;
 }
 
 .info-box .tip-button {
@@ -800,7 +805,7 @@ img.close-btn {
 	float: right;
 	height:  18px;
 	width:  18px;
-	margin-top: 1px;
+	/* margin-top: 1px; */
 	margin-right: -3px;
 	cursor: pointer;
 	border-radius: 4px;
@@ -1227,13 +1232,16 @@ div.basemap-style-btn.active img {
 	top: 23px;
 }
 
-#save-preference {
-	display: none;
+.inline-checkbox {
 	position: relative;
 	top: 1px;
 	left: 5px;
 }
 
+#save-preference {
+	/* display: none; */
+}
+
 .nav-sub-menu {
 	position: absolute;
 	z-index: -1; 
@@ -1316,25 +1324,32 @@ div.basemap-style-btn.active img {
 	background: white;
 }
 
-.save-menu-btn {
+/*.save-menu-btn {
 	display: inline-block;
 	border-radius: 4px;
 	border: 1px solid #aaa;
 	font-size: 12px;
 	margin-left: 2px;
 	padding: 1px 2px 3px 2px;
-}
+}*/
 
-.save-menu-link,
 .save-menu-btn {
-	cursor: pointer;
+	display: inline-block;
+	font-size: 12px;
+	margin-top: 2px;
 }
 
 .save-menu-btn:hover {
-	background: #e6f7ff;
 	color: black;
 }
 
+.save-menu-link,
+.save-menu-btn {
+	cursor: pointer;
+}
+
+
+
 .save-menu-link:hover {
 	/* background: #e6f7ff; */
 	font-weight: bold;