feat(runtime-core): type and attr fallthrough support for emits option

packages/runtime-core/__tests__/rendererAttrsFallthrough.spec.ts

@@ -8,7 +8,8 @@ import {
   onUpdated,
   defineComponent,
   openBlock,
-  createBlock
+  createBlock,
+  FunctionalComponent
 } from '@vue/runtime-dom'
 import { mockWarn } from '@vue/shared'
 
@@ -428,4 +429,70 @@ describe('attribute fallthrough', () => {
     await nextTick()
     expect(root.innerHTML).toBe(`<div aria-hidden="false" class="barr"></div>`)
   })
+
+  it('should not let listener fallthrough when declared in emits (stateful)', () => {
+    const Child = defineComponent({
+      emits: ['click'],
+      render() {
+        return h(
+          'button',
+          {
+            onClick: () => {
+              this.$emit('click', 'custom')
+            }
+          },
+          'hello'
+        )
+      }
+    })
+
+    const onClick = jest.fn()
+    const App = {
+      render() {
+        return h(Child, {
+          onClick
+        })
+      }
+    }
+
+    const root = document.createElement('div')
+    document.body.appendChild(root)
+    render(h(App), root)
+
+    const node = root.children[0] as HTMLElement
+    node.dispatchEvent(new CustomEvent('click'))
+    expect(onClick).toHaveBeenCalledTimes(1)
+    expect(onClick).toHaveBeenCalledWith('custom')
+  })
+
+  it('should not let listener fallthrough when declared in emits (functional)', () => {
+    const Child: FunctionalComponent<{}, { click: any }> = (_, { emit }) => {
+      // should not be in props
+      expect((_ as any).onClick).toBeUndefined()
+      return h('button', {
+        onClick: () => {
+          emit('click', 'custom')
+        }
+      })
+    }
+    Child.emits = ['click']
+
+    const onClick = jest.fn()
+    const App = {
+      render() {
+        return h(Child, {
+          onClick
+        })
+      }
+    }
+
+    const root = document.createElement('div')
+    document.body.appendChild(root)
+    render(h(App), root)
+
+    const node = root.children[0] as HTMLElement
+    node.dispatchEvent(new CustomEvent('click'))
+    expect(onClick).toHaveBeenCalledTimes(1)
+    expect(onClick).toHaveBeenCalledWith('custom')
+  })
 })

packages/runtime-core/src/apiDefineComponent.ts

@@ -3,7 +3,8 @@ import {
   MethodOptions,
   ComponentOptionsWithoutProps,
   ComponentOptionsWithArrayProps,
-  ComponentOptionsWithObjectProps
+  ComponentOptionsWithObjectProps,
+  EmitsOptions
 } from './apiOptions'
 import { SetupContext, RenderFunction } from './component'
 import { ComponentPublicInstance } from './componentProxy'
@@ -39,13 +40,15 @@ export function defineComponent<Props, RawBindings = object>(
 // (uses user defined props interface)
 // return type is for Vetur and TSX support
 export function defineComponent<
-  Props,
+  Props = {},
-  RawBindings,
+  RawBindings = {},
-  D,
+  D = {},
   C extends ComputedOptions = {},
-  M extends MethodOptions = {}
+  M extends MethodOptions = {},
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string
 >(
-  options: ComponentOptionsWithoutProps<Props, RawBindings, D, C, M>
+  options: ComponentOptionsWithoutProps<Props, RawBindings, D, C, M, E, EE>
 ): {
   new (): ComponentPublicInstance<
     Props,
@@ -53,6 +56,7 @@ export function defineComponent<
     D,
     C,
     M,
+    E,
     VNodeProps & Props
   >
 }
@@ -65,12 +69,22 @@ export function defineComponent<
   RawBindings,
   D,
   C extends ComputedOptions = {},
-  M extends MethodOptions = {}
+  M extends MethodOptions = {},
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string
 >(
-  options: ComponentOptionsWithArrayProps<PropNames, RawBindings, D, C, M>
+  options: ComponentOptionsWithArrayProps<
+    PropNames,
+    RawBindings,
+    D,
+    C,
+    M,
+    E,
+    EE
+  >
 ): {
   // array props technically doesn't place any contraints on props in TSX
-  new (): ComponentPublicInstance<VNodeProps, RawBindings, D, C, M>
+  new (): ComponentPublicInstance<VNodeProps, RawBindings, D, C, M, E>
 }
 
 // overload 4: object format with object props declaration
@@ -82,9 +96,19 @@ export function defineComponent<
   RawBindings,
   D,
   C extends ComputedOptions = {},
-  M extends MethodOptions = {}
+  M extends MethodOptions = {},
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string
 >(
-  options: ComponentOptionsWithObjectProps<PropsOptions, RawBindings, D, C, M>
+  options: ComponentOptionsWithObjectProps<
+    PropsOptions,
+    RawBindings,
+    D,
+    C,
+    M,
+    E,
+    EE
+  >
 ): {
   new (): ComponentPublicInstance<
     ExtractPropTypes<PropsOptions>,
@@ -92,6 +116,7 @@ export function defineComponent<
     D,
     C,
     M,
+    E,
     VNodeProps & ExtractPropTypes<PropsOptions, false>
   >
 }

packages/runtime-core/src/apiOptions.ts

@@ -50,12 +50,14 @@ export interface ComponentOptionsBase<
   RawBindings,
   D,
   C extends ComputedOptions,
-  M extends MethodOptions
+  M extends MethodOptions,
-> extends LegacyOptions<Props, RawBindings, D, C, M>, SFCInternalOptions {
+  E extends EmitsOptions,
+  EE extends string = string
+> extends LegacyOptions<Props, D, C, M>, SFCInternalOptions {
   setup?: (
     this: void,
     props: Props,
-    ctx: SetupContext
+    ctx: SetupContext<E>
   ) => RawBindings | RenderFunction | void
   name?: string
   template?: string | object // can be a direct DOM node
@@ -75,6 +77,7 @@ export interface ComponentOptionsBase<
   components?: Record<string, PublicAPIComponent>
   directives?: Record<string, Directive>
   inheritAttrs?: boolean
+  emits?: E | EE[]
 
   // Internal ------------------------------------------------------------------
 
@@ -97,10 +100,14 @@ export type ComponentOptionsWithoutProps<
   RawBindings = {},
   D = {},
   C extends ComputedOptions = {},
-  M extends MethodOptions = {}
+  M extends MethodOptions = {},
-> = ComponentOptionsBase<Props, RawBindings, D, C, M> & {
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string
+> = ComponentOptionsBase<Props, RawBindings, D, C, M, E, EE> & {
   props?: undefined
-} & ThisType<ComponentPublicInstance<{}, RawBindings, D, C, M, Readonly<Props>>>
+} & ThisType<
+    ComponentPublicInstance<{}, RawBindings, D, C, M, E, Readonly<Props>>
+  >
 
 export type ComponentOptionsWithArrayProps<
   PropNames extends string = string,
@@ -108,10 +115,12 @@ export type ComponentOptionsWithArrayProps<
   D = {},
   C extends ComputedOptions = {},
   M extends MethodOptions = {},
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string,
   Props = Readonly<{ [key in PropNames]?: any }>
-> = ComponentOptionsBase<Props, RawBindings, D, C, M> & {
+> = ComponentOptionsBase<Props, RawBindings, D, C, M, E, EE> & {
   props: PropNames[]
-} & ThisType<ComponentPublicInstance<Props, RawBindings, D, C, M>>
+} & ThisType<ComponentPublicInstance<Props, RawBindings, D, C, M, E>>
 
 export type ComponentOptionsWithObjectProps<
   PropsOptions = ComponentObjectPropsOptions,
@@ -119,10 +128,12 @@ export type ComponentOptionsWithObjectProps<
   D = {},
   C extends ComputedOptions = {},
   M extends MethodOptions = {},
+  E extends EmitsOptions = Record<string, any>,
+  EE extends string = string,
   Props = Readonly<ExtractPropTypes<PropsOptions>>
-> = ComponentOptionsBase<Props, RawBindings, D, C, M> & {
+> = ComponentOptionsBase<Props, RawBindings, D, C, M, E, EE> & {
   props: PropsOptions
-} & ThisType<ComponentPublicInstance<Props, RawBindings, D, C, M>>
+} & ThisType<ComponentPublicInstance<Props, RawBindings, D, C, M, E>>
 
 export type ComponentOptions =
   | ComponentOptionsWithoutProps<any, any, any, any, any>
@@ -138,6 +149,8 @@ export interface MethodOptions {
   [key: string]: Function
 }
 
+export type EmitsOptions = Record<string, any> | string[]
+
 export type ExtractComputedReturns<T extends any> = {
   [key in keyof T]: T[key] extends { get: Function }
     ? ReturnType<T[key]['get']>
@@ -162,7 +175,6 @@ type ComponentInjectOptions =
 
 export interface LegacyOptions<
   Props,
-  RawBindings,
   D,
   C extends ComputedOptions,
   M extends MethodOptions

packages/runtime-core/src/component.ts

@@ -21,7 +21,7 @@ import {
 } from './errorHandling'
 import { AppContext, createAppContext, AppConfig } from './apiCreateApp'
 import { Directive, validateDirectiveName } from './directives'
-import { applyOptions, ComponentOptions } from './apiOptions'
+import { applyOptions, ComponentOptions, EmitsOptions } from './apiOptions'
 import {
   EMPTY_OBJ,
   isFunction,
@@ -52,9 +52,13 @@ export interface SFCInternalOptions {
   __hmrUpdated?: boolean
 }
 
-export interface FunctionalComponent<P = {}> extends SFCInternalOptions {
+export interface FunctionalComponent<
-  (props: P, ctx: SetupContext): VNodeChild
+  P = {},
+  E extends EmitsOptions = Record<string, any>
+> extends SFCInternalOptions {
+  (props: P, ctx: SetupContext<E>): any
   props?: ComponentPropsOptions<P>
+  emits?: E | (keyof E)[]
   inheritAttrs?: boolean
   displayName?: string
 }
@@ -92,12 +96,29 @@ export const enum LifecycleHooks {
   ERROR_CAPTURED = 'ec'
 }
 
-export type Emit = (event: string, ...args: unknown[]) => any[]
+type UnionToIntersection<U> = (U extends any
+  ? (k: U) => void
+  : never) extends ((k: infer I) => void)
+  ? I
+  : never
 
-export interface SetupContext {
+export type Emit<
+  Options = Record<string, any>,
+  Event extends keyof Options = keyof Options
+> = Options extends any[]
+  ? (event: Options[0], ...args: any[]) => unknown[]
+  : UnionToIntersection<
+      {
+        [key in Event]: Options[key] extends ((...args: infer Args) => any)
+          ? (event: key, ...args: Args) => unknown[]
+          : (event: key, ...args: any[]) => unknown[]
+      }[Event]
+    >
+
+export interface SetupContext<E = Record<string, any>> {
   attrs: Data
   slots: Slots
-  emit: Emit
+  emit: Emit<E>
 }
 
 export type RenderFunction = {
@@ -248,7 +269,7 @@ export function createComponentInstance(
     rtc: null,
     ec: null,
 
-    emit: (event, ...args): any[] => {
+    emit: (event: string, ...args: any[]): any[] => {
       const props = instance.vnode.props || EMPTY_OBJ
       let handler = props[`on${event}`] || props[`on${capitalize(event)}`]
       if (!handler && event.indexOf('update:') === 0) {
@@ -303,9 +324,8 @@ export function setupComponent(
   isSSR = false
 ) {
   isInSSRComponentSetup = isSSR
-  const propsOptions = instance.type.props
   const { props, children, shapeFlag } = instance.vnode
-  resolveProps(instance, props, propsOptions)
+  resolveProps(instance, props)
   resolveSlots(instance, children)
 
   // setup stateful logic

packages/runtime-core/src/componentProps.ts

@@ -13,10 +13,13 @@ import {
   PatchFlags,
   makeMap,
   isReservedProp,
-  EMPTY_ARR
+  EMPTY_ARR,
+  ShapeFlags,
+  isOn
 } from '@vue/shared'
 import { warn } from './warning'
 import { Data, ComponentInternalInstance } from './component'
+import { EmitsOptions } from './apiOptions'
 
 export type ComponentPropsOptions<P = Data> =
   | ComponentObjectPropsOptions<P>
@@ -103,15 +106,17 @@ type NormalizedPropsOptions = [Record<string, NormalizedProp>, string[]]
 
 export function resolveProps(
   instance: ComponentInternalInstance,
-  rawProps: Data | null,
+  rawProps: Data | null
-  _options: ComponentPropsOptions | void
 ) {
+  const _options = instance.type.props
   const hasDeclaredProps = !!_options
   if (!rawProps && !hasDeclaredProps) {
+    instance.props = instance.attrs = EMPTY_OBJ
     return
   }
 
   const { 0: options, 1: needCastKeys } = normalizePropsOptions(_options)!
+  const emits = normalizeEmitsOptions(instance.type.emits)
   const props: Data = {}
   let attrs: Data | undefined = undefined
 
@@ -139,20 +144,18 @@ export function resolveProps(
       }
       // prop option names are camelized during normalization, so to support
       // kebab -> camel conversion here we need to camelize the key.
-      if (hasDeclaredProps) {
+      let camelKey
-        const camelKey = camelize(key)
+      if (hasDeclaredProps && hasOwn(options, (camelKey = camelize(key)))) {
-        if (hasOwn(options, camelKey)) {
+        setProp(camelKey, value)
-          setProp(camelKey, value)
+      } else if (!emits || !isListener(emits, key)) {
-        } else {
+        // Any non-declared (either as a prop or an emitted event) props are put
-          // Any non-declared props are put into a separate `attrs` object
+        // into a separate `attrs` object for spreading. Make sure to preserve
-          // for spreading. Make sure to preserve original key casing
+        // original key casing
-          ;(attrs || (attrs = {}))[key] = value
+        ;(attrs || (attrs = {}))[key] = value
-        }
-      } else {
-        setProp(key, value)
       }
     }
   }
+
   if (hasDeclaredProps) {
     // set default values & cast booleans
     for (let i = 0; i < needCastKeys.length; i++) {
@@ -186,15 +189,16 @@ export function resolveProps(
         validateProp(key, props[key], opt, !hasOwn(props, key))
       }
     }
-  } else {
-    // if component has no declared props, $attrs === $props
-    attrs = props
   }
 
   // in case of dynamic props, check if we need to delete keys from
   // the props proxy
   const { patchFlag } = instance.vnode
-  if (propsProxy && (patchFlag === 0 || patchFlag & PatchFlags.FULL_PROPS)) {
+  if (
+    hasDeclaredProps &&
+    propsProxy &&
+    (patchFlag === 0 || patchFlag & PatchFlags.FULL_PROPS)
+  ) {
     const rawInitialProps = toRaw(propsProxy)
     for (const key in rawInitialProps) {
       if (!hasOwn(props, key)) {
@@ -206,15 +210,18 @@ export function resolveProps(
   // lock readonly
   lock()
 
-  instance.props = props
+  if (
+    instance.vnode.shapeFlag & ShapeFlags.FUNCTIONAL_COMPONENT &&
+    !hasDeclaredProps
+  ) {
+    // functional component with optional props: use attrs as props
+    instance.props = attrs || EMPTY_OBJ
+  } else {
+    instance.props = props
+  }
   instance.attrs = attrs || EMPTY_OBJ
 }
 
-const normalizationMap = new WeakMap<
-  ComponentPropsOptions,
-  NormalizedPropsOptions
->()
-
 function validatePropName(key: string) {
   if (key[0] !== '$') {
     return true
@@ -230,10 +237,10 @@ export function normalizePropsOptions(
   if (!raw) {
     return EMPTY_ARR as any
   }
-  if (normalizationMap.has(raw)) {
+  if ((raw as any)._n) {
-    return normalizationMap.get(raw)!
+    return (raw as any)._n
   }
-  const options: NormalizedPropsOptions[0] = {}
+  const normalized: NormalizedPropsOptions[0] = {}
   const needCastKeys: NormalizedPropsOptions[1] = []
   if (isArray(raw)) {
     for (let i = 0; i < raw.length; i++) {
@@ -242,7 +249,7 @@ export function normalizePropsOptions(
       }
       const normalizedKey = camelize(raw[i])
       if (validatePropName(normalizedKey)) {
-        options[normalizedKey] = EMPTY_OBJ
+        normalized[normalizedKey] = EMPTY_OBJ
       }
     }
   } else {
@@ -253,7 +260,7 @@ export function normalizePropsOptions(
       const normalizedKey = camelize(key)
       if (validatePropName(normalizedKey)) {
         const opt = raw[key]
-        const prop: NormalizedProp = (options[normalizedKey] =
+        const prop: NormalizedProp = (normalized[normalizedKey] =
           isArray(opt) || isFunction(opt) ? { type: opt } : opt)
         if (prop) {
           const booleanIndex = getTypeIndex(Boolean, prop.type)
@@ -269,9 +276,38 @@ export function normalizePropsOptions(
       }
     }
   }
-  const normalized: NormalizedPropsOptions = [options, needCastKeys]
+  const normalizedEntry: NormalizedPropsOptions = [normalized, needCastKeys]
-  normalizationMap.set(raw, normalized)
+  Object.defineProperty(raw, '_n', { value: normalizedEntry })
-  return normalized
+  return normalizedEntry
+}
+
+function normalizeEmitsOptions(
+  options: EmitsOptions | undefined
+): Record<string, any> | undefined {
+  if (!options) {
+    return
+  } else if (isArray(options)) {
+    if ((options as any)._n) {
+      return (options as any)._n
+    }
+    const normalized: Record<string, null> = {}
+    options.forEach(key => (normalized[key] = null))
+    Object.defineProperty(options, '_n', normalized)
+    return normalized
+  } else {
+    return options
+  }
+}
+
+function isListener(emits: Record<string, any>, key: string): boolean {
+  if (!isOn(key)) {
+    return false
+  }
+  const eventName = key.slice(2)
+  return (
+    hasOwn(emits, eventName) ||
+    hasOwn(emits, eventName[0].toLowerCase() + eventName.slice(1))
+  )
 }
 
 // use function string name to check type constructors

packages/runtime-core/src/componentProxy.ts

@@ -7,7 +7,8 @@ import {
   ComponentOptionsBase,
   ComputedOptions,
   MethodOptions,
-  resolveMergedOptions
+  resolveMergedOptions,
+  EmitsOptions
 } from './apiOptions'
 import { ReactiveEffect, UnwrapRef } from '@vue/reactivity'
 import { warn } from './warning'
@@ -26,6 +27,7 @@ export type ComponentPublicInstance<
   D = {}, // return from data()
   C extends ComputedOptions = {},
   M extends MethodOptions = {},
+  E extends EmitsOptions = {},
   PublicProps = P
 > = {
   $: ComponentInternalInstance
@@ -36,9 +38,9 @@ export type ComponentPublicInstance<
   $slots: Slots
   $root: ComponentInternalInstance | null
   $parent: ComponentInternalInstance | null
-  $emit: Emit
+  $emit: Emit<E>
   $el: any
-  $options: ComponentOptionsBase<P, B, D, C, M>
+  $options: ComponentOptionsBase<P, B, D, C, M, E>
   $forceUpdate: ReactiveEffect
   $nextTick: typeof nextTick
   $watch: typeof instanceWatch

packages/runtime-core/src/renderer.ts

@@ -15,7 +15,6 @@ import {
 import {
   ComponentInternalInstance,
   createComponentInstance,
-  Component,
   Data,
   setupComponent
 } from './component'
@@ -1249,7 +1248,7 @@ function baseCreateRenderer(
     nextVNode.component = instance
     instance.vnode = nextVNode
     instance.next = null
-    resolveProps(instance, nextVNode.props, (nextVNode.type as Component).props)
+    resolveProps(instance, nextVNode.props)
     resolveSlots(instance, nextVNode.children)
   }
 

test-dts/defineComponent.test-d.tsx

@@ -177,35 +177,35 @@ describe('with object props', () => {
   expectError(<MyComponent b="foo" dd={{ n: 'string' }} ddd={['foo']} />)
 })
 
-describe('type inference w/ optional props declaration', () => {
+// describe('type inference w/ optional props declaration', () => {
-  const MyComponent = defineComponent({
+//   const MyComponent = defineComponent({
-    setup(_props: { msg: string }) {
+//     setup(_props: { msg: string }) {
-      return {
+//       return {
-        a: 1
+//         a: 1
-      }
+//       }
-    },
+//     },
-    render() {
+//     render() {
-      expectType<string>(this.$props.msg)
+//       expectType<string>(this.$props.msg)
-      // props should be readonly
+//       // props should be readonly
-      expectError((this.$props.msg = 'foo'))
+//       expectError((this.$props.msg = 'foo'))
-      // should not expose on `this`
+//       // should not expose on `this`
-      expectError(this.msg)
+//       expectError(this.msg)
-      expectType<number>(this.a)
+//       expectType<number>(this.a)
-      return null
+//       return null
-    }
+//     }
-  })
+//   })
 
-  expectType<JSX.Element>(<MyComponent msg="foo" />)
+//   expectType<JSX.Element>(<MyComponent msg="foo" />)
-  expectError(<MyComponent />)
+//   expectError(<MyComponent />)
-  expectError(<MyComponent msg={1} />)
+//   expectError(<MyComponent msg={1} />)
-})
+// })
 
-describe('type inference w/ direct setup function', () => {
+// describe('type inference w/ direct setup function', () => {
-  const MyComponent = defineComponent((_props: { msg: string }) => {})
+//   const MyComponent = defineComponent((_props: { msg: string }) => {})
-  expectType<JSX.Element>(<MyComponent msg="foo" />)
+//   expectType<JSX.Element>(<MyComponent msg="foo" />)
-  expectError(<MyComponent />)
+//   expectError(<MyComponent />)
-  expectError(<MyComponent msg={1} />)
+//   expectError(<MyComponent msg={1} />)
-})
+// })
 
 describe('type inference w/ array props declaration', () => {
   defineComponent({
@@ -320,3 +320,57 @@ describe('defineComponent', () => {
     })
   })
 })
+
+describe('emits', () => {
+  // Note: for TSX inference, ideally we want to map emits to onXXX props,
+  // but that requires type-level string constant concatenation as suggested in
+  // https://github.com/Microsoft/TypeScript/issues/12754
+
+  // The workaround for TSX users is instead of using emits, declare onXXX props
+  // and call them instead. Since `v-on:click` compiles to an `onClick` prop,
+  // this would also support other users consuming the component in templates
+  // with `v-on` listeners.
+
+  // with object emits
+  defineComponent({
+    emits: {
+      click: (n: number) => typeof n === 'number',
+      input: (b: string) => null
+    },
+    setup(props, { emit }) {
+      emit('click', 1)
+      emit('input', 'foo')
+      expectError(emit('nope'))
+      expectError(emit('click'))
+      expectError(emit('click', 'foo'))
+      expectError(emit('input'))
+      expectError(emit('input', 1))
+    },
+    created() {
+      this.$emit('click', 1)
+      this.$emit('input', 'foo')
+      expectError(this.$emit('nope'))
+      expectError(this.$emit('click'))
+      expectError(this.$emit('click', 'foo'))
+      expectError(this.$emit('input'))
+      expectError(this.$emit('input', 1))
+    }
+  })
+
+  // with array emits
+  defineComponent({
+    emits: ['foo', 'bar'],
+    setup(props, { emit }) {
+      emit('foo')
+      emit('foo', 123)
+      emit('bar')
+      expectError(emit('nope'))
+    },
+    created() {
+      this.$emit('foo')
+      this.$emit('foo', 123)
+      this.$emit('bar')
+      expectError(this.$emit('nope'))
+    }
+  })
+})

test-dts/functionalComponent.test-d.tsx

@@ -0,0 +1,39 @@
+import { expectError, expectType } from 'tsd'
+import { FunctionalComponent } from './index'
+
+// simple function signature
+const Foo = (props: { foo: number }) => props.foo
+
+// TSX
+expectType<JSX.Element>(<Foo foo={1} />)
+expectError(<Foo />)
+expectError(<Foo foo="bar" />)
+
+// Explicit signature with props + emits
+const Bar: FunctionalComponent<
+  { foo: number },
+  { update: (value: number) => void }
+> = (props, { emit }) => {
+  expectType<number>(props.foo)
+
+  emit('update', 123)
+  expectError(emit('nope'))
+  expectError(emit('update'))
+  expectError(emit('update', 'nope'))
+}
+
+// assigning runtime options
+Bar.props = {
+  foo: Number
+}
+expectError((Bar.props = { foo: String }))
+
+Bar.emits = {
+  update: value => value > 1
+}
+expectError((Bar.emits = { baz: () => void 0 }))
+
+// TSX
+expectType<JSX.Element>(<Bar foo={1} />)
+expectError(<Bar />)
+expectError(<Bar foo="bar" />)