Draft.js Fiddle

A fiddle to repro issues in Draft.js

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.3.0/react.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.3.0/react-dom.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.1/immutable.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/draft-js/0.7.0/Draft.js"></script>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/draft-js/0.7.0/Draft.css">
<div id="react-root"></div>

CSS

body {
  font-family: Helvetica, sans-serif;
}

.container-root {
  border: 1px solid black;
  padding: 5px;
  margin: 5px;
}

Babel + JSX

const {Editor, EditorState} = Draft;

class Container extends React.Component {
  constructor(props) {
	  super(props)
    this.state = {
      editorState: EditorState.createEmpty()
    }
  }
  
  render() {
    const { editorState } = this.state
  	const currentBlockKey = editorState.getSelection().getStartKey()
  	const currentBlockIndex = editorState.getCurrentContent().getBlockMap()
    	.keySeq().findIndex(k => k === currentBlockKey)

		return (
      <div className="container-root">
        Current line number: {currentBlockIndex + 1}
        <hr />
        <Editor 
          placeholder="Write something"
          editorState={this.state.editorState}
          onChange={this._handleChange}
        />
      </div>
    );
  }
  _handleChange = (editorState) => {
    this.setState({ editorState })
  }
}

ReactDOM.render(<Container />, document.getElementById('react-root'))