diff --git a/src/index.js b/src/index.js
--- a/src/index.js
+++ b/src/index.js
@@ -384,6 +384,8 @@ class Dayjs {
   }
 }
 
+dayjs.prototype = Dayjs.prototype
+
 dayjs.extend = (plugin, option) => {
   plugin(option, Dayjs, dayjs)
   return dayjs

diff --git a/test/constructor.test.js b/test/constructor.test.js
new file mode 100644
--- /dev/null
+++ b/test/constructor.test.js
@@ -0,0 +1,18 @@
+import MockDate from 'mockdate'
+import dayjs from '../src'
+
+beforeEach(() => {
+  MockDate.set(new Date())
+})
+
+afterEach(() => {
+  MockDate.reset()
+})
+
+it('supports instanceof dayjs', () => {
+  expect(dayjs() instanceof dayjs).toBeTruthy()
+})
+
+it('does not break isDayjs', () => {
+  expect(dayjs.isDayjs(dayjs())).toBeTruthy()
+})

dayjs() instanceof dayjs = false
Hi,
I tried to replace moment with dayjs,but some old code have this condition:
```
if (aMomentDate instanceof moment) {
//do something
}
```
if this meet dayjs, it would be false and the `if` can not be enter.

after I found the problem , I tried this:
```
moment() instanceof moment
//true
```
and this:
```
dayjs() instanceof dayjs
//false
```

So, is it necessary to be fixed?


@wslx520 Thanks.

Just added `dayjs.isDayjs` API to deal with this in release 1.6.9. So that you could do:
```javascript
if (dayjs.isDayjs( aDayjsObject)) {
// then
}
```

But I have no idea about how to make `dayjs() instanceof dayjs` to be true. Any one could help?
jQuery also: $() instanceOf $ // true
some important source code from it maybe could give you some ideas:
@iamkun:
```javascript
jQuery = function( selector, context ) {
// equals return self
    return new jQuery.fn.init( selector, context, rootjQuery );
},
jQuery.fn = jQuery.prototype = { //fn = prototype
//important
    constructor: jQuery,
    init: function( selector, context, rootjQuery ) {
        ...
// this is actually return a instance of self
        return this;
    }
    ...
}
//important
jQuery.fn.init.prototype = jQuery.fn;
```
@wslx520 Thanks. I'll try. 

And at the moment, you could use `isDayjs ` instead.