static-cms/packages/netlify-cms-widget-code/src/CodePreview.js

import React from 'react';
import PropTypes from 'prop-types';
import { Map } from 'immutable';
import { isString } from 'lodash';
import { WidgetPreviewContainer } from 'netlify-cms-ui-default';

const toValue = (value, field) => {
  if (isString(value)) {
    return value;
  }
  if (Map.isMap(value)) {
    return value.get(field.getIn(['keys', 'code'], 'code'), '');
  }
  return '';
};

const CodePreview = props => (
  <WidgetPreviewContainer>
    <pre>
      <code>{toValue(props.value, props.field)}</code>
    </pre>
  </WidgetPreviewContainer>
);

CodePreview.propTypes = {
  value: PropTypes.node,
};

export default CodePreview;