feat(props): allow defining a required prop as null

src/core/util/props.js

@@ -149,19 +149,21 @@ function assertProp (
 
 const simpleCheckRE = /^(String|Number|Boolean|Function|Symbol|BigInt)$/
 
-function assertType (value: any, type: Function, vm: ?Component): {
+function assertType (value: any, type: Function | null, vm: ?Component): {
   valid: boolean;
   expectedType: string;
 } {
   let valid
-  const expectedType = getType(type)
+  const expectedType = type ? getType(type) : 'null'
   if (simpleCheckRE.test(expectedType)) {
     const t = typeof value
     valid = t === expectedType.toLowerCase()
     // for primitive wrapper objects
     if (!valid && t === 'object') {
       valid = value instanceof type
     }
+  } else if (expectedType === 'null') {
+    valid = value === null
   } else if (expectedType === 'Object') {
     valid = isPlainObject(value)
   } else if (expectedType === 'Array') {
@@ -210,7 +212,7 @@ function getTypeIndex (type, expectedTypes): number {
 
 function getInvalidTypeMessage (name, value, expectedTypes) {
   let message = `Invalid prop: type check failed for prop "${name}".` +
-    ` Expected ${expectedTypes.map(capitalize).join(', ')}`
+    ` Expected ${expectedTypes.map(type => type === 'null' ? type : capitalize(type)).join(', ')}`
   const expectedType = expectedTypes[0]
   const receivedType = toRawType(value)
   // check if we need to specify expected value

test/unit/features/options/props.spec.js

@@ -331,6 +331,51 @@ describe('Options props', () => {
       makeInstance(null, true)
       expect(console.error.calls.count()).toBe(0)
     })
+
+    it('required + null accepts anything', () => {
+      makeInstance({}, null, null, true)
+      makeInstance(null, null, null, true)
+      makeInstance(1, null, null, true)
+      makeInstance('foo', null, null, true)
+      makeInstance(() => {}, null, null, true)
+      makeInstance(undefined, null, null, true)
+      expect(console.error.calls.count()).toBe(0)
+    })
+
+    it('required + nullable object', () => {
+      makeInstance(null, [Object, null], null, true)
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance({}, [Object, null], null, true)
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance(undefined, [Object, null], null, true)
+      expect('Expected Object, null').toHaveBeenWarned()
+    })
+
+    it('non required + nullable object', () => {
+      makeInstance(null, [Object, null])
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance({}, [Object, null])
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance(undefined, [Object, null])
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance('hello', [Object, null])
+      expect('Expected Object, null').toHaveBeenWarned()
+    })
+
+    it('required + nullable non object', () => {
+      makeInstance(null, [String, null], null, true)
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance('foo', [String, null], null, true)
+      expect(console.error.calls.count()).toBe(0)
+      makeInstance(undefined, [String, null], null, true)
+      expect('Expected String, null').toHaveBeenWarned()
+    })
+
+    it('non-required nullable string fails with object', () => {
+      makeInstance({}, [String, null])
+      expect('Expected String, null').toHaveBeenWarned()
+    })
+
   })
 
   it('should work with v-bind', () => {