Add basic test for node env (#1378)

* Add test for node env

* use immutable

* add is server flag checking

test/use-swr-immutable.test.tsx

@@ -19,6 +19,7 @@ describe('useSWR - immutable', () => {
       useData()
       return null
     }
+
     function Page() {
       const [showComponent, setShowComponent] = useState(false)
       const { data } = useData()

test/use-swr-node-env.test.tsx

@@ -0,0 +1,42 @@
+/**
+ * @jest-environment node
+ */
+
+import React from 'react'
+import { renderToString } from 'react-dom/server'
+import useSWR from '../src'
+import useSWRImmutable from '../immutable'
+import { createKey } from './utils'
+import { IS_SERVER } from '../src/utils/env'
+
+describe('useSWR', () => {
+  it('env IS_SERVER is true in node env', () => {
+    expect(IS_SERVER).toBe(true)
+  })
+
+  it('should render fallback if provided on server side', async () => {
+    const key = createKey()
+    const useData = () => useSWR(key, k => k, { fallbackData: 'fallback' })
+
+    function Page() {
+      const { data } = useData()
+      return <p>{data}</p>
+    }
+
+    const string = renderToString(<Page />)
+    expect(string).toContain('fallback')
+  })
+
+  it('should not revalidate useSWRImmutable on server side', async () => {
+    const key = createKey()
+    const useData = () => useSWRImmutable(key, k => k)
+
+    function Page() {
+      const { data } = useData()
+      return <p>{data || 'empty'}</p>
+    }
+
+    const string = renderToString(<Page />)
+    expect(string).toContain('empty')
+  })
+})