diff --git a/core/client.js b/core/client.js
index 596f756f..0e48996b 100644
--- a/core/client.js
+++ b/core/client.js
@@ -1,19 +1,30 @@
 "use strict";
 
-var gSocket;
-var gCredentials;
+let gSocket;
+let gCredentials;
 
-var gCurrentFile;
-var gFiles = {};
-var gApp = {files: {}};
-var gEditor;
-var gSplit;
-var gGraphs = {};
-var gTimeSeries = {};
-var gParentApp;
+let gCurrentFile;
+let gFiles = {};
+let gApp = {files: {}};
+let gEditor;
+let gSplit;
+let gGraphs = {};
+let gTimeSeries = {};
+let gParentApp;
+let gOriginalInput;
 
-var kErrorColor = "#dc322f";
-var kStatusColor = "#fff";
+let kErrorColor = "#dc322f";
+let kStatusColor = "#fff";
+
+/* Functions that server-side app code can call through app.setDocument()-style
+ * calls. */
+const k_api = {
+	setDocument: {args: ['content'], func: api_setDocument},
+	postMessage: {args: ['message'], func: api_postMessage},
+	error: {args: ['error'], func: api_error},
+	localStorageSet: {args: ['key', 'value'], func: api_localStorageSet},
+	localStorageGet: {args: ['key'], func: api_localStorageGet},
+};
 
 window.addEventListener("keydown", function(event) {
 	if (event.keyCode == 83 && (event.altKey || event.ctrlKey)) {
@@ -35,13 +46,13 @@ function ensureLoaded(nodes, callback) {
 		return;
 	}
 
-	var search = nodes.shift();
-	var head = document.head;
-	var found = false;
-	for (var i = 0; i < head.childNodes.length; i++) {
+	let search = nodes.shift();
+	let head = document.head;
+	let found = false;
+	for (let i = 0; i < head.childNodes.length; i++) {
 		if (head.childNodes[i].tagName == search.tagName) {
-			var match = true;
-			for (var attribute in search.attributes) {
+			let match = true;
+			for (let attribute in search.attributes) {
 				if (head.childNodes[i].attributes[attribute].value != search.attributes[attribute]) {
 					match = false;
 				}
@@ -55,11 +66,11 @@ function ensureLoaded(nodes, callback) {
 	if (found) {
 		ensureLoaded(nodes, callback);
 	} else {
-		var node = document.createElement(search.tagName);
+		let node = document.createElement(search.tagName);
 		node.onreadystatechange = node.onload = function() {
 			ensureLoaded(nodes, callback);
 		};
-		for (var attribute in search.attributes) {
+		for (let attribute in search.attributes) {
 			node.setAttribute(attribute, search.attributes[attribute]);
 		}
 		head.insertBefore(node, head.firstChild);
@@ -133,8 +144,8 @@ function trace() {
 			}
 			return response.arrayBuffer();
 		}).then(function(data) {
-			var perfetto = window.open('/perfetto/');
-			var done = false;
+			let perfetto = window.open('/perfetto/');
+			let done = false;
 			if (perfetto) {
 				function message_handler(message) {
 					if (message.data == 'PONG') {
@@ -232,8 +243,8 @@ function load(path) {
 			});
 		}
 		gFiles = {};
-		var isApp = false;
-		var promises = [];
+		let isApp = false;
+		let promises = [];
 
 		if (json && json['type'] == 'tildefriends-app') {
 			isApp = true;
@@ -248,7 +259,7 @@ function load(path) {
 		}
 		if (!isApp) {
 			document.getElementById("editPane").style.display = 'flex';
-			var text = '// New script.\n';
+			let text = '// New script.\n';
 			gCurrentFile = 'app.js';
 			gFiles[gCurrentFile] = {
 				doc: new CodeMirror.Doc(text, guessMode(gCurrentFile)),
@@ -280,9 +291,9 @@ function save(save_to) {
 		gFiles[gCurrentFile].doc = gEditor.getDoc();
 	}
 
-	var save_path = save_to;
+	let save_path = save_to;
 	if (!save_path) {
-		var name = document.getElementById("name");
+		let name = document.getElementById("name");
 		if (name && name.value) {
 			save_path = name.value;
 		} else {
@@ -290,7 +301,7 @@ function save(save_to) {
 		}
 	}
 
-	var promises = [];
+	let promises = [];
 	for (let name of Object.keys(gFiles)) {
 		let file = gFiles[name];
 		if (file.doc.isClean(file.generation)) {
@@ -318,7 +329,7 @@ function save(save_to) {
 	}
 
 	return Promise.all(promises).then(function() {
-		var app = {
+		let app = {
 			type: "tildefriends-app",
 			files: Object.fromEntries(Object.keys(gFiles).map(x => [x, gFiles[x].id || gApp.files[x]])),
 		};
@@ -366,9 +377,9 @@ function pushToParent() {
 }
 
 function url() {
-	var hash = window.location.href.indexOf('#');
-	var question = window.location.href.indexOf('?');
-	var end = -1;
+	let hash = window.location.href.indexOf('#');
+	let question = window.location.href.indexOf('?');
+	let end = -1;
 	if (hash != -1 && (hash < end || end == -1))
 	{
 		end = hash;
@@ -384,13 +395,42 @@ function hash() {
 	return window.location.hash != "#" ? window.location.hash : "";
 }
 
+function api_setDocument(content) {
+	let iframe = document.getElementById("document");
+	iframe.srcdoc = content;
+}
+
+function api_postMessage(message) {
+	let iframe = document.getElementById("document");
+	iframe.contentWindow.postMessage(message, "*");
+}
+
+function api_error(error) {
+	if (error) {
+		if (typeof(error) == 'string') {
+			setStatusMessage('⚠️ ' + error, '#f00');
+		} else {
+			setStatusMessage('⚠️ ' + merror.message + '\n' + error.stack, '#f00');
+		}
+	}
+	console.log('error', error);
+}
+
+function api_localStorageSet(key, value) {
+	window.localStorage.setItem('app:' + key, value);
+}
+
+function api_localStorageGet(key, value) {
+	send({message: 'localStorage', key: key, value: window.localStorage.getItem('app:' + key)});
+}
+
 function receive(message) {
 	if (message && message.action == "session") {
 		setStatusMessage("🟢 Executing...", kStatusColor);
 		gCredentials = message.credentials;
 		gParentApp = message.parentApp;
 		updateLogin();
-		var parent_enabled = message.parentApp;
+		let parent_enabled = message.parentApp;
 		document.getElementById('push_to_parent').style.display = parent_enabled ? 'inline-block' : 'none';
 		document.getElementById('pull_from_parent').style.display = parent_enabled ? 'inline-block' : 'none';
 	} else if (message && message.action == "ready") {
@@ -402,32 +442,13 @@ function receive(message) {
 			/* Stats were opened before we connected. */
 			send({action: 'enableStats', enabled: true});
 		}
-	} else if (message && message.action == "setDocument") {
-		var iframe = document.getElementById("document");
-		iframe.srcdoc = message.content;
-	} else if (message && message.action == "postMessage") {
-		var iframe = document.getElementById("document");
-		iframe.contentWindow.postMessage(message.message, "*");
 	} else if (message && message.action == "ping") {
 		send({action: "pong"});
-	} else if (message && message.action == "error") {
-		if (message.error) {
-			if (typeof(message.error) == 'string') {
-				setStatusMessage('⚠️ ' + message.error, '#f00');
-			} else {
-				setStatusMessage('⚠️ ' + mmessage.error.message + '\n' + message.error.stack, '#f00');
-			}
-		}
-		console.log('error', message);
-	} else if (message && message.action == "localStorageSet") {
-		window.localStorage.setItem('app:' + message.key, message.value);
-	} else if (message && message.action == "localStorageGet") {
-		send({message: 'localStorage', key: message.key, value: window.localStorage.getItem('app:' + message.key)});
 	} else if (message && message.action == "print") {
 		console.log('app>', ...message.args);
 	} else if (message && message.action == "stats") {
-		var now = new Date().getTime();
-		for (var key of Object.keys(message.stats)) {
+		let now = new Date().getTime();
+		for (let key of Object.keys(message.stats)) {
 			const k_groups = {
 				rpc_in: {group: 'rpc', name: 'in'},
 				rpc_out: {group: 'rpc', name: 'out'},
@@ -499,6 +520,11 @@ function receive(message) {
 			}
 			timeseries.append(now, message.stats[key]);
 		}
+	} else if (message && message.action) {
+		let api = k_api[message.action];
+		if (api) {
+			api.func(...api.args.map(x => message[x]));
+		}
 	}
 }
 
@@ -515,7 +541,7 @@ function keyEvent(event) {
 }
 
 function setStatusMessage(message, color) {
-	var node = document.getElementById("status");
+	let node = document.getElementById("status");
 	while (node.firstChild) {
 		node.removeChild(node.firstChild);
 	}
@@ -536,12 +562,12 @@ function send(value) {
 }
 
 function updateLogin() {
-	var login = document.getElementById("login");
+	let login = document.getElementById("login");
 	while (login.firstChild) {
 		login.removeChild(login.firstChild);
 	}
 
-	var a = document.createElement("a");
+	let a = document.createElement("a");
 	if (gCredentials && gCredentials.session) {
 		a.appendChild(document.createTextNode("logout " + gCredentials.session.name));
 		a.setAttribute("href", "/login/logout?return=" + encodeURIComponent(url() + hash()));
@@ -552,11 +578,10 @@ function updateLogin() {
 	login.appendChild(a);
 }
 
-var gOriginalInput;
 function dragHover(event) {
 	event.stopPropagation();
 	event.preventDefault();
-	var input = document.getElementById("input");
+	let input = document.getElementById("input");
 	if (event.type == "dragover") {
 		if (!input.classList.contains("drop")) {
 			input.classList.add("drop");
@@ -570,17 +595,17 @@ function dragHover(event) {
 }
 
 function fixImage(sourceData, maxWidth, maxHeight, callback) {
-	var result = sourceData;
-	var image = new Image();
+	let result = sourceData;
+	let image = new Image();
 	image.crossOrigin = "anonymous";
 	image.referrerPolicy = "no-referrer";
 	image.onload = function() {
 		if (image.width > maxWidth || image.height > maxHeight) {
-			var downScale = Math.min(maxWidth / image.width, maxHeight / image.height);
-			var canvas = document.createElement("canvas");
+			let downScale = Math.min(maxWidth / image.width, maxHeight / image.height);
+			let canvas = document.createElement("canvas");
 			canvas.width = image.width * downScale;
 			canvas.height = image.height * downScale;
-			var context = canvas.getContext("2d");
+			let context = canvas.getContext("2d");
 			context.clearRect(0, 0, canvas.width, canvas.height);
 			image.width = canvas.width;
 			image.height = canvas.height;
@@ -605,13 +630,13 @@ function fileDropRead(event) {
 function fileDrop(event) {
 	dragHover(event);
 
-	var done = false;
+	let done = false;
 	if (!done) {
-		var files = event.target.files || event.dataTransfer.files;
-		for (var i = 0; i < files.length; i++) {
-			var file = files[i];
+		let files = event.target.files || event.dataTransfer.files;
+		for (let i = 0; i < files.length; i++) {
+			let file = files[i];
 			if (file.type.substring(0, "image/".length) == "image/") {
-				var reader = new FileReader();
+				let reader = new FileReader();
 				reader.onloadend = fileDropRead;
 				reader.readAsDataURL(file);
 				done = true;
@@ -620,8 +645,8 @@ function fileDrop(event) {
 	}
 
 	if (!done) {
-		var html = event.dataTransfer.getData("text/html");
-		var match = /<img.*src="([^"]+)"/.exec(html);
+		let html = event.dataTransfer.getData("text/html");
+		let match = /<img.*src="([^"]+)"/.exec(html);
 		if (match) {
 			sendImage(match[1]);
 			done = true;
@@ -629,7 +654,7 @@ function fileDrop(event) {
 	}
 
 	if (!done) {
-		var text = event.dataTransfer.getData("text/plain");
+		let text = event.dataTransfer.getData("text/plain");
 		if (text) {
 			send(text);
 			done = true;
@@ -638,7 +663,7 @@ function fileDrop(event) {
 }
 
 function enableDragDrop() {
-	var body = document.body;
+	let body = document.body;
 	body.addEventListener("dragover", dragHover);
 	body.addEventListener("dragleave", dragHover);
 	body.addEventListener("drop", fileDrop);
@@ -664,7 +689,7 @@ function blur() {
 
 function message(event) {
 	if (event.data && event.data.event == "resizeMe" && event.data.width && event.data.height) {
-		var iframe = document.getElementById("iframe_" + event.data.name);
+		let iframe = document.getElementById("iframe_" + event.data.name);
 		iframe.setAttribute("width", event.data.width);
 		iframe.setAttribute("height", event.data.height);
 	} else if (event.data && event.data.action == "setHash") {
@@ -682,7 +707,7 @@ function message(event) {
 			}
 			return response.text();
 		}).then(function(text) {
-			var iframe = document.getElementById("document");
+			let iframe = document.getElementById("document");
 			iframe.contentWindow.postMessage({'storeBlobComplete': {name: event.data.blob.name, path: text, type: event.data.blob.type, context: event.data.context}}, '*');
 		});
 	} else {
@@ -719,13 +744,7 @@ function connectSocket(path) {
 			gSocket.send(JSON.stringify({
 				action: "hello",
 				path: path,
-				api: [
-					['setDocument', 'content'],
-					['postMessage', 'message'],
-					['error', 'error'],
-					['localStorageSet', 'key', 'value'],
-					['localStorageGet', 'key'],
-				],
+				api: Object.entries(k_api).map(([key, value]) => [].concat([key], value.args)),
 			}));
 		}
 		gSocket.onmessage = function(event) {
@@ -755,8 +774,8 @@ function connectSocket(path) {
 }
 
 function openFile(name) {
-	var newDoc = (name && gFiles[name]) ? gFiles[name].doc : new CodeMirror.Doc("", guessMode(name));
-	var oldDoc = gEditor.swapDoc(newDoc);
+	let newDoc = (name && gFiles[name]) ? gFiles[name].doc : new CodeMirror.Doc("", guessMode(name));
+	let oldDoc = gEditor.swapDoc(newDoc);
 	if (gFiles[gCurrentFile]) {
 		gFiles[gCurrentFile].doc = oldDoc;
 	}
@@ -770,13 +789,13 @@ function onFileClicked(event) {
 }
 
 function updateFiles() {
-	var node = document.getElementById("files");
+	let node = document.getElementById("files");
 	while (node.firstChild) {
 		node.removeChild(node.firstChild);
 	}
 
-	for (var file of Object.keys(gFiles).sort()) {
-		var li = document.createElement("li");
+	for (let file of Object.keys(gFiles).sort()) {
+		let li = document.createElement("li");
 		li.onclick = onFileClicked;
 		li.appendChild(document.createTextNode(file));
 		if (file == gCurrentFile) {
@@ -800,7 +819,7 @@ function makeNewFile(name) {
 }
 
 function newFile() {
-	var name = prompt("Name of new file:", "file.js");
+	let name = prompt("Name of new file:", "file.js");
 	if (name && !gFiles[name]) {
 		makeNewFile(name);
 	}
@@ -823,18 +842,18 @@ window.addEventListener("load", function() {
 	for (let tag of document.getElementsByTagName('a')) {
 		if (tag.accessKey) {
 			tag.classList.add('tooltip_parent');
-			var tooltip = document.createElement('div');
+			let tooltip = document.createElement('div');
 			tooltip.classList.add('tooltip');
 			if (tag.dataset.tip) {
-				var description = document.createElement('div');
+				let description = document.createElement('div');
 				description.innerText = tag.dataset.tip;
 				tooltip.appendChild(description);
 			}
-			var parts = tag.accessKeyLabel ? tag.accessKeyLabel.split('+') : [];
-			for (var i = 0; i < parts.length; i++)
+			let parts = tag.accessKeyLabel ? tag.accessKeyLabel.split('+') : [];
+			for (let i = 0; i < parts.length; i++)
 			{
-				var key = parts[i];
-				var kbd = document.createElement('kbd');
+				let key = parts[i];
+				let kbd = document.createElement('kbd');
 				kbd.innerText = key;
 				tooltip.appendChild(kbd);
 				if (i < parts.length - 1) {