diff --git a/src/compiler/compile/render_dom/invalidate.ts b/src/compiler/compile/render_dom/invalidate.ts
index 7eeb34a89400..31df2cbe3b5b 100644
--- a/src/compiler/compile/render_dom/invalidate.ts
+++ b/src/compiler/compile/render_dom/invalidate.ts
@@ -50,6 +50,7 @@ export function invalidate(renderer: Renderer, scope: Scope, node: Node, names:
 	const extra_args = tail.map(variable => get_invalidated(variable)).filter(Boolean);
 
 	if (is_store_value) {
+		// TODO: check why there are 4 parameters, but `set_store_value` only expects 3
 		return x`@set_store_value(${head.name.slice(1)}, ${node}, ${head.name}, ${extra_args})`;
 	}
 
diff --git a/src/runtime/internal/utils.ts b/src/runtime/internal/utils.ts
index 8868e38ee29f..0111c2a94a18 100644
--- a/src/runtime/internal/utils.ts
+++ b/src/runtime/internal/utils.ts
@@ -1,13 +1,16 @@
-import { Readable } from 'svelte/store';
+import { Readable, Subscriber, Invalidator, Writable } from 'svelte/store';
+import { SvelteComponent } from '..';
 
-export function noop() {}
+export function noop() { }
 
-export const identity = x => x;
+export function identity<T>(x: T): T {
+	return x;
+}
 
-export function assign<T, S>(tar: T, src: S): T & S {
+export function assign<T, S>(target: T, source: S): T & S {
 	// @ts-ignore
-	for (const k in src) tar[k] = src[k];
-	return tar as T & S;
+	for (const k in source) target[k] = source[k];
+	return target as T & S;
 }
 
 export function is_promise<T = any>(value: any): value is PromiseLike<T> {
@@ -20,15 +23,17 @@ export function add_location(element, file, line, column, char) {
 	};
 }
 
-export function run(fn) {
-	return fn();
+export function blank_object(): {} {
+	return Object.create(null);
 }
 
-export function blank_object() {
-	return Object.create(null);
+type FunctionWithoutArguments = () => unknown
+
+export function run(fn: FunctionWithoutArguments) {
+	return fn();
 }
 
-export function run_all(fns: Function[]) {
+export function run_all(fns: FunctionWithoutArguments[]) {
 	fns.forEach(run);
 }
 
@@ -36,13 +41,9 @@ export function is_function(thing: any): thing is Function {
 	return typeof thing === 'function';
 }
 
-export function safe_not_equal(a, b) {
-	return a != a ? b == b : a !== b || ((a && typeof a === 'object') || typeof a === 'function');
-}
-
-let src_url_equal_anchor;
+let src_url_equal_anchor: HTMLAnchorElement;
 
-export function src_url_equal(element_src, url) {
+export function src_url_equal(element_src: string, url: string) {
 	if (!src_url_equal_anchor) {
 		src_url_equal_anchor = document.createElement('a');
 	}
@@ -50,52 +51,55 @@ export function src_url_equal(element_src, url) {
 	return element_src === src_url_equal_anchor.href;
 }
 
-export function not_equal(a, b) {
+export function safe_not_equal(a: unknown, b: unknown) {
+	return a != a ? b == b : a !== b || ((a && typeof a === 'object') || typeof a === 'function');
+}
+
+export function not_equal(a: unknown, b: unknown) {
 	return a != a ? b == b : a !== b;
 }
 
-export function is_empty(obj) {
+export function is_empty(obj: Record<string | number | symbol, unknown>) {
 	return Object.keys(obj).length === 0;
 }
 
-export function validate_store(store, name) {
+export function validate_store<S extends Readable<unknown>>(store: S, name: string) {
 	if (store != null && typeof store.subscribe !== 'function') {
 		throw new Error(`'${name}' is not a store with a 'subscribe' method`);
 	}
 }
 
-export function subscribe(store, ...callbacks) {
+export function subscribe<T, S extends Readable<T>>(store: S, run: Subscriber<T>, invalidate?: Invalidator<T>) {
 	if (store == null) {
 		return noop;
 	}
-	const unsub = store.subscribe(...callbacks);
-	return unsub.unsubscribe ? () => unsub.unsubscribe() : unsub;
+	return store.subscribe(run, invalidate);
 }
 
-export function get_store_value<T>(store: Readable<T>): T {
-	let value;
-	subscribe(store, _ => value = _)();
+export function get_store_value<T, S extends Readable<T>>(store: S): T {
+	let value: T;
+	subscribe<T, S>(store, v => value = v)();
 	return value;
 }
 
-export function component_subscribe(component, store, callback) {
+export function component_subscribe<T, S extends Readable<T>>(component: SvelteComponent, store: S, callback: Subscriber<T>) {
 	component.$$.on_destroy.push(subscribe(store, callback));
 }
 
-export function create_slot(definition, ctx, $$scope, fn) {
+export function create_slot(definition, ctx, $$scope, fn: Function) {
 	if (definition) {
 		const slot_ctx = get_slot_context(definition, ctx, $$scope, fn);
 		return definition[0](slot_ctx);
 	}
 }
 
-function get_slot_context(definition, ctx, $$scope, fn) {
+function get_slot_context(definition, ctx, $$scope, fn: Function) {
 	return definition[1] && fn
 		? assign($$scope.ctx.slice(), definition[1](fn(ctx)))
 		: $$scope.ctx;
 }
 
-export function get_slot_changes(definition, $$scope, dirty, fn) {
+export function get_slot_changes(definition, $$scope, dirty, fn: Function) {
 	if (definition[2] && fn) {
 		const lets = definition[2](fn(dirty));
 
@@ -164,25 +168,27 @@ export function compute_slots(slots) {
 	return result;
 }
 
-export function once(fn) {
+export function once(fn: Function) {
 	let ran = false;
-	return function(this: any, ...args) {
+	return function (this: any, ...args: unknown[]) {
 		if (ran) return;
 		ran = true;
 		fn.call(this, ...args);
 	};
 }
 
-export function null_to_empty(value) {
-	return value == null ? '' : value;
+export function null_to_empty<T>(value: T): T extends null | undefined ? '' : T {
+	return (value == null ? '' : value) as T extends null | undefined ? '' : T;
 }
 
-export function set_store_value(store, ret, value) {
+export function set_store_value<T, S extends Writable<T>>(store: S, ret: Node, value: T) {
 	store.set(value);
 	return ret;
 }
 
-export const has_prop = (obj, prop) => Object.prototype.hasOwnProperty.call(obj, prop);
+export function has_prop<X extends {}, Y extends PropertyKey>(obj: X, prop: Y): obj is X & Record<Y, unknown> {
+	return Object.prototype.hasOwnProperty.call(obj, prop);
+}
 
 export function action_destroyer(action_result) {
 	return action_result && is_function(action_result.destroy) ? action_result.destroy : noop;
diff --git a/src/runtime/store/index.ts b/src/runtime/store/index.ts
index e947fa074028..a3132342fa94 100644
--- a/src/runtime/store/index.ts
+++ b/src/runtime/store/index.ts
@@ -10,7 +10,7 @@ export type Unsubscriber = () => void;
 export type Updater<T> = (value: T) => T;
 
 /** Cleanup logic callback. */
-type Invalidator<T> = (value?: T) => void;
+export type Invalidator<T> = (value?: T) => void;
 
 /** Start and stop notification callbacks. */
 export type StartStopNotifier<T> = (set: Subscriber<T>) => Unsubscriber | void;