diff --git a/.gitignore b/.gitignore
index c3004c1157..4d7bbc7ac3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -29,5 +29,6 @@ _actual*.*
 /site/.sessions
 /site/static/svelte-app.json
 /site/static/contributors.jpg
+/site/static/workers
 /site/scripts/svelte-app
 /site/src/routes/_contributors.js
diff --git a/package-lock.json b/package-lock.json
index e2b0fd0ffd..1c6ced1a92 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,6 +1,6 @@
 {
   "name": "svelte",
-  "version": "3.4.3",
+  "version": "3.4.4",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -168,7 +168,7 @@
     },
     "array-equal": {
       "version": "1.0.0",
-      "resolved": "http://registry.npmjs.org/array-equal/-/array-equal-1.0.0.tgz",
+      "resolved": "https://registry.npmjs.org/array-equal/-/array-equal-1.0.0.tgz",
       "integrity": "sha1-jCpe8kcv2ep0KwTHenUJO6J1fJM=",
       "dev": true
     },
@@ -528,7 +528,7 @@
     },
     "commander": {
       "version": "2.15.1",
-      "resolved": "http://registry.npmjs.org/commander/-/commander-2.15.1.tgz",
+      "resolved": "https://registry.npmjs.org/commander/-/commander-2.15.1.tgz",
       "integrity": "sha512-VlfT9F3V0v+jr4yxPc5gg9s62/fIVWsd2Bk2iD435um1NlGMYdVCq+MjcXnhYq2icNOizHr1kK+5TI6H0Hy0ag==",
       "dev": true
     },
@@ -844,9 +844,9 @@
       "dev": true
     },
     "estree-walker": {
-      "version": "0.6.0",
-      "resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.0.tgz",
-      "integrity": "sha512-peq1RfVAVzr3PU/jL31RaOjUKLoZJpObQWJJ+LgfcxDUifyLZ1RjPQZTl0pzj2uJ45b7A7XpyppXvxdEqzo4rw==",
+      "version": "0.6.1",
+      "resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
+      "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w==",
       "dev": true
     },
     "esutils": {
@@ -1307,7 +1307,7 @@
     },
     "is-builtin-module": {
       "version": "1.0.0",
-      "resolved": "http://registry.npmjs.org/is-builtin-module/-/is-builtin-module-1.0.0.tgz",
+      "resolved": "https://registry.npmjs.org/is-builtin-module/-/is-builtin-module-1.0.0.tgz",
       "integrity": "sha1-VAVy0096wxGfj3bDDLwbHgN6/74=",
       "dev": true,
       "requires": {
@@ -1712,7 +1712,7 @@
     },
     "minimist": {
       "version": "0.0.8",
-      "resolved": "http://registry.npmjs.org/minimist/-/minimist-0.0.8.tgz",
+      "resolved": "https://registry.npmjs.org/minimist/-/minimist-0.0.8.tgz",
       "integrity": "sha1-hX/Kv8M5fSYluCKCYuhqp6ARsF0=",
       "dev": true
     },
@@ -1739,7 +1739,7 @@
     },
     "mkdirp": {
       "version": "0.5.1",
-      "resolved": "http://registry.npmjs.org/mkdirp/-/mkdirp-0.5.1.tgz",
+      "resolved": "https://registry.npmjs.org/mkdirp/-/mkdirp-0.5.1.tgz",
       "integrity": "sha1-MAV0OOrGz3+MR2fzhkjWaX11yQM=",
       "dev": true,
       "requires": {
@@ -2070,7 +2070,7 @@
     },
     "path-is-absolute": {
       "version": "1.0.1",
-      "resolved": "http://registry.npmjs.org/path-is-absolute/-/path-is-absolute-1.0.1.tgz",
+      "resolved": "https://registry.npmjs.org/path-is-absolute/-/path-is-absolute-1.0.1.tgz",
       "integrity": "sha1-F0uSaHNVNP+8es5r9TpanhtcX18=",
       "dev": true
     },
@@ -2710,7 +2710,7 @@
     },
     "safe-regex": {
       "version": "1.1.0",
-      "resolved": "http://registry.npmjs.org/safe-regex/-/safe-regex-1.1.0.tgz",
+      "resolved": "https://registry.npmjs.org/safe-regex/-/safe-regex-1.1.0.tgz",
       "integrity": "sha1-QKNmnzsHfR6UPURinhV91IAjvy4=",
       "dev": true,
       "requires": {
diff --git a/package.json b/package.json
index 0be4033eb9..6d4bfb134e 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,7 @@
   "main": "index",
   "files": [
     "types",
-    "compiler.js",
+    "compiler.*",
     "register.js",
     "index.*",
     "internal",
@@ -63,7 +63,7 @@
     "c8": "^3.4.0",
     "codecov": "^3.0.0",
     "css-tree": "1.0.0-alpha22",
-    "estree-walker": "^0.6.0",
+    "estree-walker": "^0.6.1",
     "is-reference": "^1.1.1",
     "jsdom": "^12.2.0",
     "kleur": "^3.0.0",
diff --git a/site/content/examples/06-lifecycle/02-update/App.svelte b/site/content/examples/06-lifecycle/02-update/App.svelte
index 373b6dcb6a..3eb2dceb52 100644
--- a/site/content/examples/06-lifecycle/02-update/App.svelte
+++ b/site/content/examples/06-lifecycle/02-update/App.svelte
@@ -1,5 +1,5 @@
 <script>
-	import Eliza from 'elizanode';
+	import Eliza from 'elizabot';
 	import { beforeUpdate, afterUpdate } from 'svelte';
 
 	let div;
diff --git a/site/content/tutorial/07-lifecycle/03-update/app-a/App.svelte b/site/content/tutorial/07-lifecycle/03-update/app-a/App.svelte
index 2081b6ab8c..75fce63ca2 100644
--- a/site/content/tutorial/07-lifecycle/03-update/app-a/App.svelte
+++ b/site/content/tutorial/07-lifecycle/03-update/app-a/App.svelte
@@ -1,5 +1,5 @@
 <script>
-	import Eliza from 'elizanode';
+	import Eliza from 'elizabot';
 	import { beforeUpdate, afterUpdate } from 'svelte';
 
 	let div;
diff --git a/site/content/tutorial/07-lifecycle/03-update/app-b/App.svelte b/site/content/tutorial/07-lifecycle/03-update/app-b/App.svelte
index 373b6dcb6a..3eb2dceb52 100644
--- a/site/content/tutorial/07-lifecycle/03-update/app-b/App.svelte
+++ b/site/content/tutorial/07-lifecycle/03-update/app-b/App.svelte
@@ -1,5 +1,5 @@
 <script>
-	import Eliza from 'elizanode';
+	import Eliza from 'elizabot';
 	import { beforeUpdate, afterUpdate } from 'svelte';
 
 	let div;
diff --git a/site/package-lock.json b/site/package-lock.json
index b61cee722e..0127c84d53 100644
--- a/site/package-lock.json
+++ b/site/package-lock.json
@@ -1362,13 +1362,15 @@
       }
     },
     "@sveltejs/svelte-repl": {
-      "version": "0.0.11",
-      "resolved": "https://registry.npmjs.org/@sveltejs/svelte-repl/-/svelte-repl-0.0.11.tgz",
-      "integrity": "sha512-F284f8qaUs1rp8akqWXcB6oovlaso7qmsUz1rqm80FwUKLffjYIWy2a1p6+Yo1kRy6Q+fW8kj21JLEqv7pjOwA==",
+      "version": "0.1.2",
+      "resolved": "https://registry.npmjs.org/@sveltejs/svelte-repl/-/svelte-repl-0.1.2.tgz",
+      "integrity": "sha512-OHIj01ly4+/WwNb0zGqLSQL7POSs2BPx47CzryVsbu0omVDe7olYTJnCYV4ePUENnlL5JzZihzvlvxmVGP6knw==",
       "dev": true,
       "requires": {
         "codemirror": "^5.45.0",
-        "sourcemap-codec": "^1.4.4"
+        "estree-walker": "^0.6.0",
+        "sourcemap-codec": "^1.4.4",
+        "yootils": "0.0.15"
       }
     },
     "@types/estree": {
@@ -1799,9 +1801,9 @@
       "dev": true
     },
     "codemirror": {
-      "version": "5.46.0",
-      "resolved": "https://registry.npmjs.org/codemirror/-/codemirror-5.46.0.tgz",
-      "integrity": "sha512-3QpMge0vg4QEhHW3hBAtCipJEWjTJrqLLXdIaWptJOblf1vHFeXLNtFhPai/uX2lnFCehWNk4yOdaMR853Z02w==",
+      "version": "5.47.0",
+      "resolved": "https://registry.npmjs.org/codemirror/-/codemirror-5.47.0.tgz",
+      "integrity": "sha512-kV49Fr+NGFHFc/Imsx6g180hSlkGhuHxTSDDmDHOuyln0MQYFLixDY4+bFkBVeCEiepYfDimAF/e++9jPJk4QA==",
       "dev": true
     },
     "collection-visit": {
diff --git a/site/package.json b/site/package.json
index d9de5a217e..ec429f54ab 100644
--- a/site/package.json
+++ b/site/package.json
@@ -3,9 +3,10 @@
   "version": "1.0.0",
   "description": "Docs and examples for Svelte",
   "scripts": {
-    "dev": "sapper dev",
+    "dev": "npm run copy-workers && sapper dev",
+    "copy-workers": "rm -rf static/workers && cp -r node_modules/@sveltejs/svelte-repl/workers static",
     "migrate": "node-pg-migrate -r dotenv/config",
-    "sapper": "sapper build --legacy",
+    "sapper": "npm run copy-workers && sapper build --legacy",
     "update_shimport": "cp node_modules/shimport/index.js __sapper__/build/client/shimport@0.0.14.js",
     "update": "node scripts/update_template.js && node scripts/get-contributors.js",
     "start": "node __sapper__/build",
@@ -38,7 +39,7 @@
     "@babel/runtime": "^7.4.4",
     "@sindresorhus/slugify": "^0.9.1",
     "@sveltejs/site-kit": "^1.0.4",
-    "@sveltejs/svelte-repl": "0.0.11",
+    "@sveltejs/svelte-repl": "^0.1.2",
     "degit": "^2.1.3",
     "dotenv": "^8.0.0",
     "eslint-plugin-svelte3": "^1.0.0",
diff --git a/site/src/components/Repl/ReplWidget.svelte b/site/src/components/Repl/ReplWidget.svelte
index 07a5814010..9d06fb7868 100644
--- a/site/src/components/Repl/ReplWidget.svelte
+++ b/site/src/components/Repl/ReplWidget.svelte
@@ -116,6 +116,7 @@
 		{#if process.browser}
 			<Repl
 				bind:this={repl}
+				workersUrl="workers"
 				fixed={mobile}
 				{svelteUrl}
 				{rollupUrl}
diff --git a/site/src/routes/examples/index.svelte b/site/src/routes/examples/index.svelte
index a5c816561b..b21d0c773d 100644
--- a/site/src/routes/examples/index.svelte
+++ b/site/src/routes/examples/index.svelte
@@ -102,6 +102,7 @@
 		<div class="repl-container" class:loading={isLoading}>
 			<Repl
 				bind:this={repl}
+				workersUrl="workers"
 				{svelteUrl}
 				{rollupUrl}
 				orientation={replOrientation}
diff --git a/site/src/routes/repl/[id]/index.svelte b/site/src/routes/repl/[id]/index.svelte
index 4c5a22d6da..f2a3c02173 100644
--- a/site/src/routes/repl/[id]/index.svelte
+++ b/site/src/routes/repl/[id]/index.svelte
@@ -206,6 +206,7 @@
 		<div class="viewport" class:offset={checked}>
 			<Repl
 				bind:this={repl}
+				workersUrl="workers"
 				{svelteUrl}
 				{rollupUrl}
 				{relaxed}
diff --git a/site/src/routes/tutorial/[slug]/index.svelte b/site/src/routes/tutorial/[slug]/index.svelte
index 34c9d38f79..1f166d70bf 100644
--- a/site/src/routes/tutorial/[slug]/index.svelte
+++ b/site/src/routes/tutorial/[slug]/index.svelte
@@ -302,6 +302,7 @@
 		<div class="tutorial-repl">
 			<Repl
 				bind:this={repl}
+				workersUrl="workers"
 				{svelteUrl}
 				{rollupUrl}
 				orientation={mobile ? 'columns' : 'rows'}
diff --git a/site/src/server.js b/site/src/server.js
index 45f3e16dae..2adf3c5ad1 100644
--- a/site/src/server.js
+++ b/site/src/server.js
@@ -2,11 +2,12 @@ import sirv from 'sirv';
 import * as sapper from '@sapper/server';
 import { API } from './backend/auth';
 
-const { PORT=3000 } = process.env;
+const { PORT = 3000 } = process.env;
 
 API()
 	.use(
 		sirv('static', {
+			dev: process.env.NODE_ENV === 'development',
 			setHeaders(res) {
 				res.setHeader('Access-Control-Allow-Origin', '*');
 				res.hasHeader('Cache-Control') || res.setHeader('Cache-Control', 'max-age=600'); // 10min default
diff --git a/src/runtime/animate/index.ts b/src/runtime/animate/index.ts
index cf64cd060a..f5500020cc 100644
--- a/src/runtime/animate/index.ts
+++ b/src/runtime/animate/index.ts
@@ -1,7 +1,22 @@
 import { cubicOut } from 'svelte/easing';
 import { is_function } from 'svelte/internal';
 
-export function flip(node, animation, params) {
+// todo: same as Transition, should it be shared?
+export interface AnimationConfig {
+	delay?: number,
+	duration?: number,
+	easing?: (t: number) => number,
+	css?: (t: number, u: number) => string,
+	tick?: (t: number, u: number) => void
+}
+
+interface FlipParams {
+	delay: number;
+	duration: number | ((len: number) => number);
+	easing: (t: number) => number,
+}
+
+export function flip(node: Element, animation: { from: DOMRect, to: DOMRect }, params: FlipParams): AnimationConfig {
 	const style = getComputedStyle(node);
 	const transform = style.transform === 'none' ? '' : style.transform;
 
diff --git a/src/runtime/internal/animations.ts b/src/runtime/internal/animations.ts
index 8d52b3ca7a..5a055ba6d2 100644
--- a/src/runtime/internal/animations.ts
+++ b/src/runtime/internal/animations.ts
@@ -1,18 +1,28 @@
 import { identity as linear, noop, now } from './utils';
 import { loop } from './loop';
 import { create_rule, delete_rule } from './style_manager';
+import { AnimationConfig } from '../animate';
 
-export function create_animation(node, from, fn, params) {
+
+//todo: documentation says it is DOMRect, but in IE it would be ClientRect
+type PositionRect = DOMRect|ClientRect;
+
+type AnimationFn = (node: Element, { from, to }: { from: PositionRect, to: PositionRect }, params: any) => AnimationConfig;
+
+export function create_animation(node: Element & ElementCSSInlineStyle, from: PositionRect, fn: AnimationFn, params) {
 	if (!from) return noop;
 
 	const to = node.getBoundingClientRect();
 	if (from.left === to.left && from.right === to.right && from.top === to.top && from.bottom === to.bottom) return noop;
 
+
 	const {
 		delay = 0,
 		duration = 300,
 		easing = linear,
+		// @ts-ignore todo: should this be separated from destructuring? Or start/end added to public api and documentation?
 		start: start_time = now() + delay,
+		// @ts-ignore todo:
 		end = start_time + duration,
 		tick = noop,
 		css
@@ -67,7 +77,7 @@ export function create_animation(node, from, fn, params) {
 	return stop;
 }
 
-export function fix_position(node) {
+export function fix_position(node: Element & ElementCSSInlineStyle) {
 	const style = getComputedStyle(node);
 
 	if (style.position !== 'absolute' && style.position !== 'fixed') {
diff --git a/src/runtime/internal/dom.ts b/src/runtime/internal/dom.ts
index 293040ce20..edf394e4da 100644
--- a/src/runtime/internal/dom.ts
+++ b/src/runtime/internal/dom.ts
@@ -53,8 +53,8 @@ export function object_without_properties<T,K extends keyof T>(obj:T, exclude: K
 	return target;
 }
 
-export function svg_element(name:string):SVGElement {
-	return document.createElementNS('http://www.w3.org/2000/svg', name);
+export function svg_element<K extends keyof SVGElementTagNameMap>(name:K):SVGElement {
+	return document.createElementNS<K>('http://www.w3.org/2000/svg', name);
 }
 
 export function text(data:string) {
@@ -95,7 +95,7 @@ export function attr(node: Element, attribute: string, value?: string) {
 	else node.setAttribute(attribute, value);
 }
 
-export function set_attributes(node: HTMLElement, attributes: { [x: string]: string; }) {
+export function set_attributes(node: Element & ElementCSSInlineStyle, attributes: { [x: string]: string; }) {
 	for (const key in attributes) {
 		if (key === 'style') {
 			node.style.cssText = attributes[key];
diff --git a/src/runtime/internal/loop.ts b/src/runtime/internal/loop.ts
index 73f118c234..c41d72ed74 100644
--- a/src/runtime/internal/loop.ts
+++ b/src/runtime/internal/loop.ts
@@ -1,6 +1,6 @@
 import { now, raf } from './utils';
 
-export interface Task { abort(): void; promise: Promise<undefined> }
+export interface Task { abort(): void; promise: Promise<void> }
 
 const tasks = new Set();
 let running = false;
@@ -32,7 +32,7 @@ export function loop(fn: (number)=>void): Task {
 	}
 
 	return {
-		promise: new Promise<undefined>(fulfil => {
+		promise: new Promise<void>(fulfil => {
 			tasks.add(task = [fn, fulfil]);
 		}),
 		abort() {
diff --git a/src/runtime/internal/ssr.ts b/src/runtime/internal/ssr.ts
index aaa391b4dc..c80b4d9f45 100644
--- a/src/runtime/internal/ssr.ts
+++ b/src/runtime/internal/ssr.ts
@@ -1,5 +1,6 @@
 import { set_current_component, current_component } from './lifecycle';
 import { run_all, blank_object } from './utils';
+import { Readable } from 'svelte/store';
 
 export const invalid_attribute_name_character = /[\s'">/=\u{FDD0}-\u{FDEF}\u{FFFE}\u{FFFF}\u{1FFFE}\u{1FFFF}\u{2FFFE}\u{2FFFF}\u{3FFFE}\u{3FFFF}\u{4FFFE}\u{4FFFF}\u{5FFFE}\u{5FFFF}\u{6FFFE}\u{6FFFF}\u{7FFFE}\u{7FFFF}\u{8FFFE}\u{8FFFF}\u{9FFFE}\u{9FFFF}\u{AFFFE}\u{AFFFF}\u{BFFFE}\u{BFFFF}\u{CFFFE}\u{CFFFF}\u{DFFFE}\u{DFFFF}\u{EFFFE}\u{EFFFF}\u{FFFFE}\u{FFFFF}\u{10FFFE}\u{10FFFF}]/u;
 // https://html.spec.whatwg.org/multipage/syntax.html#attributes-2
@@ -113,7 +114,7 @@ export function create_ssr_component(fn) {
 	};
 }
 
-export function get_store_value(store) {
+export function get_store_value<T>(store: Readable<T>): T | undefined {
 	let value;
 	store.subscribe(_ => value = _)();
 	return value;
diff --git a/src/runtime/internal/style_manager.ts b/src/runtime/internal/style_manager.ts
index 9ef1b12d1c..2721200627 100644
--- a/src/runtime/internal/style_manager.ts
+++ b/src/runtime/internal/style_manager.ts
@@ -6,7 +6,7 @@ let active = 0;
 let current_rules = {};
 
 // https://github.com/darkskyapp/string-hash/blob/master/index.js
-function hash(str) {
+function hash(str: string) {
 	let hash = 5381;
 	let i = str.length;
 
@@ -14,7 +14,7 @@ function hash(str) {
 	return hash >>> 0;
 }
 
-export function create_rule(node, a, b, duration, delay, ease, fn, uid = 0) {
+export function create_rule(node: Element & ElementCSSInlineStyle, a: number, b: number, duration: number, delay: number, ease: (t: number) => number, fn: (t: number, u: number) => string, uid: number = 0) {
 	const step = 16.666 / duration;
 	let keyframes = '{\n';
 
@@ -44,7 +44,7 @@ export function create_rule(node, a, b, duration, delay, ease, fn, uid = 0) {
 	return name;
 }
 
-export function delete_rule(node, name?) {
+export function delete_rule(node: Element & ElementCSSInlineStyle, name?: string) {
 	node.style.animation = (node.style.animation || '')
 		.split(', ')
 		.filter(name
diff --git a/src/runtime/internal/transitions.ts b/src/runtime/internal/transitions.ts
index 04942e3d16..f3e8414e5d 100644
--- a/src/runtime/internal/transitions.ts
+++ b/src/runtime/internal/transitions.ts
@@ -1,10 +1,11 @@
-import { identity as linear, noop, now, run_all } from './utils';
+import { identity as linear, is_function, noop, now, run_all } from './utils';
 import { loop } from './loop';
 import { create_rule, delete_rule } from './style_manager';
 import { custom_event } from './dom';
 import { add_render_callback } from './scheduler';
+import { TransitionConfig } from '../transition';
 
-let promise;
+let promise: Promise<void>|null;
 
 function wait() {
 	if (!promise) {
@@ -17,7 +18,7 @@ function wait() {
 	return promise;
 }
 
-function dispatch(node, direction, kind) {
+function dispatch(node: Element, direction: boolean, kind: 'start' | 'end') {
 	node.dispatchEvent(custom_event(`${direction ? 'intro' : 'outro'}${kind}`));
 }
 
@@ -39,8 +40,9 @@ export function check_outros() {
 export function on_outro(callback) {
 	outros.callbacks.push(callback);
 }
+type TransitionFn = (node: Element, params: any) => TransitionConfig;
 
-export function create_in_transition(node, fn, params) {
+export function create_in_transition(node: Element & ElementCSSInlineStyle, fn: TransitionFn, params: any) {
 	let config = fn(node, params);
 	let running = false;
 	let animation_name;
@@ -95,7 +97,7 @@ export function create_in_transition(node, fn, params) {
 
 			delete_rule(node);
 
-			if (typeof config === 'function') {
+			if (is_function(config)) {
 				config = config();
 				wait().then(go);
 			} else {
@@ -116,7 +118,7 @@ export function create_in_transition(node, fn, params) {
 	};
 }
 
-export function create_out_transition(node, fn, params) {
+export function create_out_transition(node: Element & ElementCSSInlineStyle, fn: TransitionFn, params: any) {
 	let config = fn(node, params);
 	let running = true;
 	let animation_name;
@@ -163,8 +165,9 @@ export function create_out_transition(node, fn, params) {
 		});
 	}
 
-	if (typeof config === 'function') {
+	if (is_function(config)) {
 		wait().then(() => {
+			// @ts-ignore
 			config = config();
 			go();
 		});
@@ -186,7 +189,7 @@ export function create_out_transition(node, fn, params) {
 	};
 }
 
-export function create_bidirectional_transition(node, fn, params, intro) {
+export function create_bidirectional_transition(node: Element & ElementCSSInlineStyle, fn: TransitionFn, params: any, intro: boolean) {
 	let config = fn(node, params);
 
 	let t = intro ? 0 : 1;
@@ -295,8 +298,9 @@ export function create_bidirectional_transition(node, fn, params, intro) {
 
 	return {
 		run(b) {
-			if (typeof config === 'function') {
+			if (is_function(config)) {
 				wait().then(() => {
+					// @ts-ignore
 					config = config();
 					go(b);
 				});
diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index c562cd02b6..7997a25b1d 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -2,13 +2,14 @@ export function noop() {}
 
 export const identity = x => x;
 
-export function assign(tar, src) {
+export function assign<T, S>(tar:T, src:S): T & S {
+	// @ts-ignore
 	for (const k in src) tar[k] = src[k];
-	return tar;
+	return tar as T & S;
 }
 
-export function is_promise(value) {
-	return value && typeof value.then === 'function';
+export function is_promise<T = any>(value: any): value is PromiseLike<T> {
+	return value && typeof value === 'object' && typeof value.then === 'function';
 }
 
 export function add_location(element, file, line, column, char) {
@@ -29,7 +30,7 @@ export function run_all(fns) {
 	fns.forEach(run);
 }
 
-export function is_function(thing) {
+export function is_function(thing: any): thing is Function {
 	return typeof thing === 'function';
 }
 
diff --git a/src/runtime/motion/spring.ts b/src/runtime/motion/spring.ts
index 3977c08f79..f99e8db41f 100644
--- a/src/runtime/motion/spring.ts
+++ b/src/runtime/motion/spring.ts
@@ -2,9 +2,18 @@ import { Readable, writable } from 'svelte/store';
 import { loop, now, Task } from 'svelte/internal';
 import { is_date } from './utils';
 
-function tick_spring(ctx, last_value, current_value, target_value) {
+interface TickContext<T> {
+	inv_mass: number;
+	dt: number;
+	opts: Spring<T>;
+	settled: boolean
+}
+
+function tick_spring<T>(ctx: TickContext<T>, last_value: T, current_value: T, target_value: T):T {
 	if (typeof current_value === 'number' || is_date(current_value)) {
+		// @ts-ignore
 		const delta = target_value - current_value;
+		// @ts-ignore
 		const velocity = (current_value - last_value) / (ctx.dt||1/60); // guard div by 0
 		const spring = ctx.opts.stiffness * delta;
 		const damper = ctx.opts.damping * velocity;
@@ -15,16 +24,20 @@ function tick_spring(ctx, last_value, current_value, target_value) {
 			return target_value; // settled
 		} else {
 			ctx.settled = false; // signal loop to keep ticking
+			// @ts-ignore
 			return is_date(current_value) ?
 				new Date(current_value.getTime() + d) : current_value + d;
 		}
 	} else if (Array.isArray(current_value)) {
+		// @ts-ignore
 		return current_value.map((_, i) =>
 			tick_spring(ctx, last_value[i], current_value[i], target_value[i]));
 	} else if (typeof current_value === 'object') {
 		const next_value = {};
 		for (const k in current_value)
+			// @ts-ignore
 			next_value[k] = tick_spring(ctx, last_value[k], current_value[k], target_value[k]);
+		// @ts-ignore
 		return next_value;
 	} else {
 		throw new Error(`Cannot spring ${typeof current_value} values`);
@@ -37,17 +50,22 @@ interface SpringOpts {
 	precision?: number,
 }
 
-type SpringUpdateOpts = { hard?: any; soft?: string | number | boolean; };
+interface SpringUpdateOpts {
+	hard?: any;
+	soft?: string | number | boolean;
+}
+
+type Updater<T> = (target_value: T, value: T) => T;
 
-interface Spring<T=any> extends Readable<T>{
-	set: (new_value: T, opts?: SpringUpdateOpts) => (Promise<T> | Promise<T>);
+interface Spring<T> extends Readable<T>{
+	set: (new_value: T, opts?: SpringUpdateOpts) => Promise<void>;
+	update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
 	precision: number;
-	update: (fn, opts: SpringUpdateOpts) => Promise<T>;
 	damping: number;
 	stiffness: number
 }
 
-export function spring<T=any>(value: T, opts: SpringOpts = {}) {
+export function spring<T=any>(value: T, opts: SpringOpts = {}): Spring<T> {
 	const store = writable(value);
 	const { stiffness = 0.15, damping = 0.8, precision = 0.01 } = opts;
 
@@ -61,7 +79,7 @@ export function spring<T=any>(value: T, opts: SpringOpts = {}) {
 	let inv_mass_recovery_rate = 0;
 	let cancel_task = false;
 
-	function set(new_value: any, opts: SpringUpdateOpts={}) {
+	function set(new_value: T, opts: SpringUpdateOpts={}): Promise<void> {
 		target_value = new_value;
 		const token = current_token = {};
 
@@ -91,7 +109,7 @@ export function spring<T=any>(value: T, opts: SpringOpts = {}) {
 
 				inv_mass = Math.min(inv_mass + inv_mass_recovery_rate, 1);
 
-				const ctx = {
+				const ctx: TickContext<T> = {
 					inv_mass,
 					opts: spring,
 					settled: true, // tick_spring may signal false
@@ -116,14 +134,14 @@ export function spring<T=any>(value: T, opts: SpringOpts = {}) {
 		});
 	}
 
-	const spring: Spring = {
+	const spring = {
 		set,
 		update: (fn, opts:SpringUpdateOpts) => set(fn(target_value, value), opts),
 		subscribe: store.subscribe,
 		stiffness,
 		damping,
 		precision
-	};
+	} as Spring<T>;
 
 	return spring;
 }
diff --git a/src/runtime/motion/tweened.ts b/src/runtime/motion/tweened.ts
index 3b2d0ef5a2..ea3f2cdd86 100644
--- a/src/runtime/motion/tweened.ts
+++ b/src/runtime/motion/tweened.ts
@@ -1,6 +1,6 @@
-import { writable } from 'svelte/store'; // eslint-disable-line import/no-unresolved
-import { assign, loop, now } from 'svelte/internal'; // eslint-disable-line import/no-unresolved
-import { linear } from 'svelte/easing'; // eslint-disable-line import/no-unresolved
+import { Readable, writable } from 'svelte/store';
+import { assign, loop, now, Task } from 'svelte/internal';
+import { linear } from 'svelte/easing';
 import { is_date } from './utils';
 
 function get_interpolator(a, b) {
@@ -54,13 +54,28 @@ function get_interpolator(a, b) {
 	throw new Error(`Cannot interpolate ${type} values`);
 }
 
-export function tweened(value, defaults = {}) {
+interface Options<T> {
+	delay?: number;
+	duration?: number | ((from: T, to: T) => number)
+	easing?: (t: number) => number;
+	interpolate?: (a: T, b: T) => (t: number) => T
+}
+
+type Updater<T> = (target_value: T, value: T) => T;
+
+interface Tweened<T> extends Readable<T> {
+	set(value: T, opts: Options<T>): Promise<void>;
+
+	update(updater: Updater<T>, opts: Options<T>): Promise<void>;
+}
+
+export function tweened<T>(value: T, defaults: Options<T> = {}):Tweened<T> {
 	const store = writable(value);
 
-	let task;
+	let task: Task;
 	let target_value = value;
 
-	function set(new_value, opts) {
+	function set(new_value: T, opts: Options<T>) {
 		target_value = new_value;
 
 		let previous_task = task;
@@ -97,6 +112,7 @@ export function tweened(value, defaults = {}) {
 				return false;
 			}
 
+			// @ts-ignore
 			store.set(value = fn(easing(elapsed / duration)));
 			return true;
 		});
@@ -106,7 +122,7 @@ export function tweened(value, defaults = {}) {
 
 	return {
 		set,
-		update: (fn, opts) => set(fn(target_value, value), opts),
+		update: (fn, opts:Options<T>) => set(fn(target_value, value), opts),
 		subscribe: store.subscribe
 	};
 }
diff --git a/src/runtime/motion/utils.ts b/src/runtime/motion/utils.ts
index f8bfcce14c..2eab8baa2f 100644
--- a/src/runtime/motion/utils.ts
+++ b/src/runtime/motion/utils.ts
@@ -1,3 +1,3 @@
-export function is_date(obj: any) {
+export function is_date(obj: any): obj is Date {
 	return Object.prototype.toString.call(obj) === '[object Date]';
 }
diff --git a/src/runtime/transition/index.ts b/src/runtime/transition/index.ts
index 20619ee131..b3f4f21288 100644
--- a/src/runtime/transition/index.ts
+++ b/src/runtime/transition/index.ts
@@ -1,10 +1,23 @@
 import { cubicOut, cubicInOut } from 'svelte/easing';
 import { assign, is_function } from 'svelte/internal';
 
-export function fade(node, {
+export interface TransitionConfig {
+	delay?: number,
+	duration?: number,
+	easing?: (t: number) => number,
+	css?: (t: number, u: number) => string,
+	tick?: (t: number, u: number) => void
+}
+
+interface FadeParams {
+	delay: number;
+	duration: number;
+}
+
+export function fade(node: Element, {
 	delay = 0,
 	duration = 400
-}) {
+}: FadeParams): TransitionConfig {
 	const o = +getComputedStyle(node).opacity;
 
 	return {
@@ -14,14 +27,23 @@ export function fade(node, {
 	};
 }
 
-export function fly(node, {
+interface FlyParams {
+	delay: number;
+	duration: number;
+	easing: (t: number)=>number,
+	x: number;
+	y: number;
+	opacity: number;
+}
+
+export function fly(node: Element, {
 	delay = 0,
 	duration = 400,
 	easing = cubicOut,
 	x = 0,
 	y = 0,
 	opacity = 0
-}) {
+}: FlyParams): TransitionConfig {
 	const style = getComputedStyle(node);
 	const target_opacity = +style.opacity;
 	const transform = style.transform === 'none' ? '' : style.transform;
@@ -38,11 +60,17 @@ export function fly(node, {
 	};
 }
 
-export function slide(node, {
+interface SlideParams {
+	delay: number;
+	duration: number;
+	easing: (t: number)=>number,
+}
+
+export function slide(node: Element, {
 	delay = 0,
 	duration = 400,
 	easing = cubicOut
-}) {
+}: SlideParams): TransitionConfig {
 	const style = getComputedStyle(node);
 	const opacity = +style.opacity;
 	const height = parseFloat(style.height);
@@ -70,13 +98,21 @@ export function slide(node, {
 	};
 }
 
-export function scale(node, {
+interface ScaleParams {
+	delay: number;
+	duration: number;
+	easing: (t: number)=>number,
+	start: number;
+	opacity: number;
+}
+
+export function scale(node: Element, {
 	delay = 0,
 	duration = 400,
 	easing = cubicOut,
 	start = 0,
 	opacity = 0
-}) {
+}: ScaleParams): TransitionConfig {
 	const style = getComputedStyle(node);
 	const target_opacity = +style.opacity;
 	const transform = style.transform === 'none' ? '' : style.transform;
@@ -95,12 +131,19 @@ export function scale(node, {
 	};
 }
 
-export function draw(node, {
+interface DrawParams {
+	delay: number;
+	speed: number;
+	duration: number | ((len: number) => number);
+	easing: (t: number) => number,
+}
+
+export function draw(node: SVGElement & { getTotalLength(): number }, {
 	delay = 0,
 	speed,
 	duration,
 	easing = cubicInOut
-}) {
+}: DrawParams): TransitionConfig {
 	const len = node.getTotalLength();
 
 	if (duration === undefined) {
@@ -121,11 +164,21 @@ export function draw(node, {
 	};
 }
 
-export function crossfade({ fallback, ...defaults }) {
-	const to_receive = new Map();
-	const to_send = new Map();
+interface CrossfadeParams {
+	delay: number;
+	duration: number | ((len: number) => number);
+	easing: (t: number) => number,
+}
+
+type ClientRectMap = Map<any, { rect: ClientRect }>;
+
+export function crossfade({ fallback, ...defaults }: CrossfadeParams & {
+	fallback: (node: Element, params: CrossfadeParams, intro: boolean)=> TransitionConfig
+}) {
+	const to_receive: ClientRectMap = new Map();
+	const to_send: ClientRectMap = new Map();
 
-	function crossfade(from, node, params) {
+	function crossfade(from: ClientRect, node: Element, params: CrossfadeParams):TransitionConfig {
 		const {
 			delay = 0,
 			duration = d => Math.sqrt(d) * 30,
@@ -152,8 +205,8 @@ export function crossfade({ fallback, ...defaults }) {
 		};
 	}
 
-	function transition(items, counterparts, intro) {
-		return (node, params) => {
+	function transition(items: ClientRectMap, counterparts: ClientRectMap, intro: boolean) {
+		return (node: Element, params: CrossfadeParams & { key: any }) => {
 			items.set(params.key, {
 				rect: node.getBoundingClientRect()
 			});