renderStylesToString doesn't work with custom emotion instances.

[whmountains]

• emotion version: 9.0.2
• react version: 16.2.0

I'm using react-static.

My React component:

// custom emotion instance
import styled from '../lib/react-emotion'

const Title = styled.h1`
  color: limegreen;
`
export default () => {
    <Title style={{ textAlign: 'center' }}>Welcome!</Title>
)

Server rendering code:

const html = renderStylesToString(render(<Comp />))
console.log(global.__EMOTION_CTX)
console.log(html)
return html

console.log(html) prints this: (redacted to just show the important part)

<style data-emotion-css="aoqeef-Title"></style>
<h1 class="css-aoqeef-Title ehamwg80">Welcome!</h1>

The style tag is empty. Not good.

To further debug this, I looked at emotion's context object:

{ __SECRET_EMOTION__: 
   { registered: 
      { 'css-aoqeef-Title': 'color:limegreen;text-align:center;/*# sourceMappingURL=data:application/json;charset=utf-8;base64,eyJ2ZXJzaW9uIjozL...snip...' },
     inserted: 
      { 'aoqeef-Title': '' },
     sheet: _StyleSheet { isSpeedy: true, tags: [], ctr: 0, opts: {} },
     stylis: { [Function: F] use: [Function: da], set: [Function: ea] },
     nonce: undefined,
     key: 'css' } }

Notice that inserted.aoqeef-Title is an empty string. Something tells me it should contain css. https://github.com/emotion-js/emotion/blob/master/packages/create-emotion-server/src/inline.js#L18 seems to confirm this.

So where is the empty string coming from? I believe it comes from here: https://github.com/emotion-js/emotion/blob/master/packages/create-emotion/src/index.js#L249-L262

This snippet in particular seems to be the culprit:

current = ''
stylis(scope, styles)
caches.inserted[name] = current

I was able to make everything work by re-writing it this way:

caches.inserted[name] = stylis(scope, styles)

I'm not familiar with emotion's internals, so this may just be a symtom of a mis-configuration on my part.

[whmountains]

Update: my original analysis as to the root cause was wrong. The snippet I pointed to isn't the issue after all.

The problem has something to do with using renderStylesToString with a custom emotion instance.

[whmountains]

Closing in favor of #603