diff --git a/lib/one.js b/lib/one.js
index 384dca84..f02adb32 100644
--- a/lib/one.js
+++ b/lib/one.js
@@ -1,16 +1,20 @@
 // @flow
 
-'use strict'
-
-// TODO: Find a way to type HTMLElement with $one.
-type OneElement = any
+type OneElement = HTMLElement & {
+  $one: Component,
+  on?: Function,
+  if?: Function,
+  for?: Function,
+  [string]: Function
+}
 
 type Props = Array<string> | { [string]: mixed }
-type State = { [string]: mixed }
+type State = Object
 type Computed = { [string]: Function }
+type Key = string | number | boolean
 
 type Config = {
-  key?: string,
+  key?: Key,
   uid?: number,
   name?: string,
   template?: TrustedHTML,
@@ -22,11 +26,20 @@ type Config = {
   store?: Store,
   setup: Function,
   created?: Function,
-  components?: Array<Config>
+  components?: Array<Config>,
+  loop?: {
+    parent: OneElement,
+    el: OneElement,
+    items: string,
+    key: ?Key,
+    fn: Function
+  }
 }
 
+type Component = Omit<Config, 'setup'>
+
 type Components = {
-  [string]: Array<Omit<Config, 'setup'>>
+  [string]: Array<Component>
 }
 
 type Store = {
@@ -70,11 +83,14 @@ const store: Store = {
 
 let uid = 0
 
-function one(config: Config) {
-  const els = config.parent
-    ? config.parent.el?.querySelectorAll(config.name)
-    // FIXME: Don't use ?? '' here.
-    : document.querySelectorAll(config.name ?? '')
+function one(config: Config): void {
+  if (!config.name) {
+    throw new Error('One->one: You must define a name for your component.')
+  }
+
+  const els: NodeList<OneElement> = config.parent
+    ? (config.parent.el?.querySelectorAll(config.name): any)
+    : (document.querySelectorAll(config.name): any)
 
   if (els == null || els.length === 0) {
     return
@@ -123,9 +139,9 @@ function one(config: Config) {
   })
 }
 
-function template(config: Omit<Config, 'setup'>) {
+function template(config: Component) {
   const tmpl = document.createElement('template')
-  
+
   // FIXME: Shouldn't bail when there's no template.
   // The user could've defined the template in HTML.
   // i.e.: config.el.children...
@@ -136,8 +152,11 @@ function template(config: Omit<Config, 'setup'>) {
     throw new Error('One->template: You can only have exactly one root element in your template.')
   }
 
-  const name = config.name ?? ''
-  const clone: OneElement = tmpl.content.children[0].cloneNode(true)
+  const clone: OneElement = (tmpl.content.children[0].cloneNode(true): any)
+
+  const name = config.name
+  if (!name) return
+
   const component = components[name].find(c => c.uid === config.uid)
 
   if (!component) {
@@ -148,11 +167,11 @@ function template(config: Omit<Config, 'setup'>) {
     throw new Error('One->template: Could not find the element.')
   }
 
-  const el: OneElement = config.el
+  const el = config.el
   clone.$one = config.el?.$one
 
   for (const attr of config.el.attributes) {
-    if (config.props?.[attr.name]) continue
+    if (config.props && (config.props: Object)[attr.name]) continue
     clone.setAttribute(attr.name, attr.value)
   }
 
@@ -164,7 +183,7 @@ function template(config: Omit<Config, 'setup'>) {
   tmpl.remove()
 }
 
-function createInstance(config: Omit<Config, 'setup'>): Omit<Config, 'setup'> {
+function createInstance(config: Component): Component {
   const name = config.name
   const el = config.el
 
@@ -190,7 +209,8 @@ function createInstance(config: Omit<Config, 'setup'>): Omit<Config, 'setup'> {
     uid,
     props: {},
     state: config.state,
-    parent: config.parent
+    parent: config.parent,
+    el: config.el,
   }
 
   const props: Object = {}
@@ -203,7 +223,7 @@ function createInstance(config: Omit<Config, 'setup'>): Omit<Config, 'setup'> {
           props[prop] = val
         }
       }
-      el.$one.props[prop] = val
+      (el.$one.props: Object)[prop] = val
     })
   }
 
@@ -216,9 +236,9 @@ function createInstance(config: Omit<Config, 'setup'>): Omit<Config, 'setup'> {
   return config
 }
 
-function query(this: Omit<Config, 'setup'>, selector: string) {
-  const el = this.el?.children.length
-    ? this.el.querySelector(selector)
+function query(this: Component, selector: string) {
+  const el: ?OneElement = this.el?.children.length
+    ? (this.el.querySelector(selector): any)
     : this.el
 
   if (!el) {
@@ -232,7 +252,7 @@ function query(this: Omit<Config, 'setup'>, selector: string) {
   return el
 }
 
-function on(this: Omit<Config, 'setup'>, el: OneElement, name: string, handler: Function) {
+function on(this: Component, el: OneElement, name: string, handler: Function) {
   el[`on${name}`] = async () => {
     await handler({
       store: { get: store.get, set: store.set, length: store.length },
@@ -258,17 +278,14 @@ function on(this: Omit<Config, 'setup'>, el: OneElement, name: string, handler:
   }
 }
 
-// TOOD: Support for items that aren't an array of objects
-// i.e: [1, 2, 3] or ['foo', 'bar', 'baz']
-// How to handle the key in this case?
 function loop(
-  this: Omit<Config, 'setup'>,
+  this: Component,
   el: OneElement,
-  [items, key]: [string, string],
+  [items, key]: [string, ?Key],
   fn: Function,
   parent: ?OneElement = null
 ) {
-  const parentNode = parent ?? el.parentNode
+  const parentNode: OneElement = parent ?? (el.parentNode: any)
   const arr = this.state?.[items] ?? this.parent?.state?.[items]
 
   if (!Array.isArray(arr)) {
@@ -285,9 +302,9 @@ function loop(
   // `parent` will only be defined if it's a re-render.
   // Or, if the user defines it, which should be discouraged.
   if (parent) {
-    Array.from(parentNode.children).forEach((child) => {
-      const k = child.$one?.key
-      if (!k) { return }
+    Array.from(parentNode.children).forEach((child: any) => {
+      const k: OneElement = child.$one?.key
+      if (!k) return
       if (!arr.find((x: Object) => x[key] === k)) {
         child.remove()
       }
@@ -300,17 +317,20 @@ function loop(
     el.remove()
   }
 
-  arr.forEach((item: Object, index: number) => {
-    const clone = el.cloneNode(true)
+  const keys: Array<Key> = []
+  arr.forEach((item: Object | string | number | boolean, index: number) => {
+    const clone: OneElement = (el.cloneNode(true): any)
+    const k = getKey(item, key).toString()
 
-    if (!item[key]) {
-      throw new Error(`One->for: The key "${key}" does not exist on ${JSON.stringify(item)}`)
+    if (keys.includes(k)) {
+      throw new Error(`One->for: There are multiple items with the key "${k}" when looping "${items}".`)
     }
+    keys.push(k)
 
     el.remove()
 
     const config = {
-      key: item[key],
+      key: k,
       el: clone,
       state: item,
       store: this.store
@@ -323,8 +343,9 @@ function loop(
 
     render(clone.$one)
 
-    if (parentNode.children[index]) {
-      render(parentNode.children[index].$one)
+    const child: OneElement = (parentNode.children[index]: any)
+    if (child) {
+      render(child.$one)
     } else {
       parentNode.append(clone)
     }
@@ -335,33 +356,49 @@ function loop(
   })
 }
 
-function fi(this: Omit<Config, 'setup'>, el: OneElement) {
+function getKey(item: Object | string | number | boolean, key: ?Key) {
+  if (!key && typeof item === 'object') {
+    throw new Error(`One->for: You must define a key for "${JSON.stringify(item)}"`)
+  }
+
+  if (key && typeof item === 'object' && !item[key]) {
+    throw new Error(`One->for: The key "${key.toString()}" does not exist on ${JSON.stringify(item)}`)
+  }
+
+  if (typeof item === 'object') {
+    return item[key]
+  }
+
+  return item
+}
+
+function fi(this: Component, el: OneElement) {
   let original: ?OneElement = null
 
-  const container = document.createElement('one-if')
+  const container: OneElement = (document.createElement('one-if'): any)
   const template = document.createElement('template')
-  const sibling = el.nextSibling
+  const sibling: OneElement = (el.nextSibling: any)
 
   return {
     hide() {
-      if (original) { return }
+      if (original) return
 
       template.append(el)
       container.append(template)
 
-      sibling.parentNode.insertBefore(container, sibling)
+      sibling.parentNode?.insertBefore(container, sibling)
 
       original = container
     },
     show() {
-      if (!original) { return }
+      if (!original) return
       original.replaceWith(el)
       original = null
     }
   }
 }
 
-function computed(this: Omit<Config, 'setup'>, name: string, fn: Function) {
+function computed(this: Component, name: string, fn: Function) {
   if (this.state?.[name] != null) {
     throw new Error(`One->computed: You already have a state property called "${name}".`)
   }
@@ -383,11 +420,11 @@ function computed(this: Omit<Config, 'setup'>, name: string, fn: Function) {
   }
 }
 
-function render(config: Omit<Config, 'setup'>) {
+function render(config: Component) {
   traverse(config.el, config)
 }
 
-function traverse(el: OneElement, config: Omit<Config, 'setup'>) {
+function traverse(el: OneElement, config: Component) {
   Array.from(el.attributes).forEach(attr => {
     if (attr.name === 'o-text') {
       const val = value(attr.value, config)
@@ -398,21 +435,23 @@ function traverse(el: OneElement, config: Omit<Config, 'setup'>) {
     }
 
     if (attr.name === 'o-for') {
+      if (!el.$one?.loop) return
       const { el: elm, items, key, fn, parent } = el.$one.loop
       loop.bind(config)(elm, [items, key], fn, parent)
     }
   })
 
   if (el.children.length) {
-    Array.from(el.children).forEach(c => {
-      if (c.$one?.uid) { return }
+    const children: Array<OneElement> = (Array.from(el.children): any)
+    children.forEach((c) => {
+      if (c.$one?.uid) return
       traverse(c, config)
     })
   }
 }
 
 // TODO: Support for nested keys, like `foo.bar.baz`.
-function value(key: string, config: Omit<Config, 'setup'>) {
+function value(key: string, config: Component) {
   const fromStore = key.startsWith('$store.')
   const fromProps = key.startsWith('$props.')
 
@@ -436,6 +475,10 @@ function value(key: string, config: Omit<Config, 'setup'>) {
     throw new Error('One->value: You are trying to access a state value, but the component have no state.')
   }
 
+  if (typeof config.state !== 'object') {
+    return config.state
+  }
+
   return config.state[key]
 }