print warnings in dev mode for unused data

src/api/dev.ts

@@ -224,11 +224,8 @@ class Watcher extends EventEmitter {
 					});
 
 					this.proc.on('message', message => {
-						if (message.__sapper__ && message.event === 'basepath') {
+						if (!message.__sapper__) return;
-							this.emit('basepath', {
+						this.emit(message.event, message);
-								basepath: message.basepath
-							});
-						}
 					});
 
 					this.proc.on('exit', emitFatal);

src/cli/dev.ts

@@ -2,6 +2,7 @@ import * as path from 'path';
 import * as colors from 'ansi-colors';
 import * as child_process from 'child_process';
 import prettyMs from 'pretty-ms';
+import pb from 'pretty-bytes';
 import { dev as _dev } from '../api/dev';
 import * as events from '../api/interfaces';
 
@@ -44,6 +45,32 @@ export function dev(opts: { port: number, open: boolean }) {
 			if (event.log) console.log(event.log);
 		});
 
+		watcher.on('preload', (event) => {
+			if (event.size > 25000) {
+				console.log(colors.bold.yellow(`${event.url} — large amount of preloaded data`));
+				console.log(`${colors.bold(pb(event.size))} of data was preloaded in total, above the recommended limit of ${pb(25000)}`);
+			}
+		});
+
+		watcher.on('unused_data', (event) => {
+			console.log(colors.bold.yellow(`${event.url} — unused preloaded data`));
+			console.log(`More data was returned from \`preload\` than was used during the initial render. Consider only returning essential data.`);
+
+			event.discrepancies.forEach(discrepancy => {
+				console.log(`${colors.bold(discrepancy.file)} loaded ${colors.bold(pb(discrepancy.preloaded))}, of which ${discrepancy.rendered > 2 ? `only ${colors.bold(pb(discrepancy.rendered))}` : 'none'} was used. The following properties were not referenced:`);
+
+				const slice = discrepancy.props.length > 12
+					? discrepancy.props.slice(0, 10)
+					: discrepancy.props;
+
+				console.log(slice.map((prop: string) => `• ${prop}`).join('\n'));
+
+				if (discrepancy.props.length > slice.length) {
+					console.log(`...and ${discrepancy.props.length - slice.length} more`);
+				}
+			});
+		});
+
 		watcher.on('build', (event: events.BuildEvent) => {
 			if (event.errors.length) {
 				console.log(`${colors.bold.red(`✗ ${event.type}`)}`);

src/core/create_manifests.ts

@@ -156,6 +156,7 @@ function generate_server(
 
 							const props = [
 								`name: "${part.component.name}"`,
+								`file: "${part.component.file}"`,
 								`component: ${part.component.name}`
 							];
 

src/middleware.ts

@@ -8,6 +8,9 @@ import fetch from 'node-fetch';
 import { lookup } from './middleware/mime';
 import { locations, dev } from './config';
 import sourceMapSupport from 'source-map-support';
+import prettyBytes from 'pretty-bytes';
+import { wrap_data } from './middleware/wrap_data';
+import { list_unused_properties } from './middleware/list_unused_properties';
 
 sourceMapSupport.install();
 
@@ -290,6 +293,8 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 	const { server_routes, pages } = manifest;
 	const error_route = manifest.error;
 
+	const should_wrap_data = dev() || process.env.SAPPER_EXPORT;
+
 	function handle_error(req: Req, res: ServerResponse, statusCode: number, error: Error | string) {
 		handle_page({
 			pattern: null,
@@ -428,11 +433,6 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 				return;
 			}
 
-			const serialized = {
-				preloaded: `[${preloaded.map(data => try_serialize(data)).join(',')}]`,
-				store: store && try_serialize(store.get())
-			};
-
 			const segments = req.path.split('/').filter(Boolean);
 
 			const props: Props = {
@@ -454,6 +454,15 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 				}
 			});
 
+			// in dev and export modes, we wrap data in proxies to see
+			// how much of it is used in the initial render
+			const wrapped = should_wrap_data && wrap_data(preloaded);
+
+			// this is an easy way to 'reify' top-level values
+			const _preloaded = should_wrap_data
+				? wrapped.data.map((x: any) => x)
+				: preloaded;
+
 			let level = data.child;
 			for (let i = 0; i < page.parts.length; i += 1) {
 				const part = page.parts[i];
@@ -465,7 +474,7 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 					component: part.component,
 					props: Object.assign({}, props, {
 						params: get_params(match)
-					}, preloaded[i + 1])
+					}, _preloaded[i + 1])
 				});
 
 				level.props.child = <Props["child"]>{
@@ -484,6 +493,47 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 				.map(file => `<script src='${req.baseUrl}/client/${file}'></script>`)
 				.join('');
 
+			const unwrapped = should_wrap_data && wrapped.unwrap();
+
+			const preloaded_serialized = preloaded.map(try_serialize);
+
+			if (should_wrap_data && process.send) {
+				const discrepancies = [];
+
+				unwrapped.forEach((clone, i) => {
+					const loaded = preloaded_serialized[i];
+					if (!loaded) return;
+
+					const rendered = try_serialize(clone);
+
+					if (rendered !== loaded) {
+						const part = page.parts[i - 1];
+						const file = part ? part.file : '_layout.html';
+
+						discrepancies.push({
+							file,
+							preloaded: loaded.length,
+							rendered: rendered.length,
+							props: list_unused_properties(preloaded[i], clone)
+						});
+					}
+				});
+
+				if (discrepancies.length) {
+					process.send({
+						__sapper__: true,
+						event: 'unused_data',
+						url: req.url,
+						discrepancies
+					});
+				}
+			}
+
+			const serialized = {
+				preloaded: `[${preloaded_serialized.join(',')}]`,
+				store: store && try_serialize(store.get())
+			};
+
 			let inline_script = `__SAPPER__={${[
 				error && `error:1`,
 				`baseUrl:"${req.baseUrl}"`,
@@ -507,6 +557,13 @@ function get_page_handler(manifest: Manifest, store_getter: (req: Req) => Store)
 			res.end(body);
 
 			if (process.send) {
+				process.send({
+					__sapper__: true,
+					event: 'preload',
+					url: req.url,
+					size: serialized.preloaded.length
+				});
+
 				process.send({
 					__sapper__: true,
 					event: 'file',

src/middleware/list_unused_properties.ts

@@ -0,0 +1,34 @@
+export function list_unused_properties(all: any, used: any) {
+	const props: string[] = [];
+
+	const seen = new Set();
+
+	function walk(keypath: string, a: any, b: any) {
+		if (seen.has(a)) return;
+		seen.add(a);
+
+		if (!a || typeof a !== 'object') return;
+
+		const is_array = Array.isArray(a);
+
+		for (const key in a) {
+			const child_keypath = keypath
+				? is_array ? `${keypath}[${key}]` : `${keypath}.${key}`
+				: key;
+
+			if (hasProp.call(b, key)) {
+				const a_child = a[key];
+				const b_child = b[key];
+
+				walk(child_keypath, a_child, b_child);
+			} else {
+				props.push(child_keypath);
+			}
+		}
+	}
+
+	walk(null, all, used);
+	return props;
+}
+
+const hasProp = Object.prototype.hasOwnProperty;

src/middleware/wrap_data.ts

@@ -0,0 +1,85 @@
+type Obj = Record<string, any>;
+
+export function wrap_data(data: any) {
+	const proxies = new Map();
+	const clones = new Map();
+
+	const handler = {
+		get(target: any, property: string): any {
+			const value = target[property];
+			const intercepted = intercept(value);
+
+			const target_clone = clones.get(target);
+			const child_clone = clones.get(value);
+
+			if (target_clone && target.hasOwnProperty(property)) {
+				target_clone[property] = child_clone || value;
+			}
+
+			return intercepted;
+		},
+	};
+
+	function get_or_create_proxy(obj: any) {
+		if (!proxies.has(obj)) {
+			proxies.set(obj, new Proxy(obj, handler));
+		}
+
+		return proxies.get(obj);
+	}
+
+	function intercept(obj: any) {
+		if (clones.has(obj)) return obj;
+
+		if (obj && typeof obj === 'object') {
+			if (Array.isArray(obj)) {
+				clones.set(obj, []);
+				return get_or_create_proxy(obj);
+			}
+
+			else if (isPlainObject(obj)) {
+				clones.set(obj, {});
+				return get_or_create_proxy(obj);
+			}
+		}
+
+		clones.set(obj, obj);
+		return obj;
+	}
+
+	return {
+		data: intercept(data),
+		unwrap: () => {
+			return clones.get(data);
+		}
+	};
+}
+
+const objectProtoOwnPropertyNames = Object.getOwnPropertyNames(Object.prototype).sort().join('\0')
+
+function isPlainObject(obj: any) {
+	const proto = Object.getPrototypeOf(obj);
+
+	if (
+		proto !== Object.prototype &&
+		proto !== null &&
+		Object.getOwnPropertyNames(proto).sort().join('\0') !== objectProtoOwnPropertyNames
+	) {
+		return false;
+	}
+
+	if (Object.getOwnPropertySymbols(obj).length > 0) {
+		return false;
+	}
+
+	return true;
+}
+
+
+function pick(obj: Obj, props: string[]) {
+	const picked: Obj = {};
+	props.forEach(prop => {
+		picked[prop] = obj[prop];
+	});
+	return picked;
+}