How to concatenate two JSX fragment or variables or string and component (in Reactjs)?

Use arrays:

let lineComponent = <Line key={line.client_id} line={line}/>;
if (line.created_at) {
  return [
    <div key="date" className="date-line"><strong>{line.created_at}</strong></div>,
    lineComponent,
  ];
} else {
  return chat_line;
}

Or use fragments:

import createFragment from "react-addons-create-fragment";

let lineComponent = <Line key={line.client_id} line={line}/>;
if (line.created_at) {
  return createFragment({
    date: <div className="date-line"><strong>{line.created_at}</strong></div>,
    lineComponent: lineComponent,
  });
} else {
  return chat_line;
}

In both cases, you have to provide keys for React. In case of array, you set key directly on element. Regarding fragments, you provide key:element pairs.

NOTE: When returning from render method, you can only return single element, or NULL. Examples provided are invalid in that case.

For React Native, I prefer this technique:

1. pro: in contrast to the array technique you don't have to artificially create keys
2. con: requires the overhead of a containing element (e.g., View, below)

jsx = <Text>first</Text>;
jsx = <View>{jsx}<Text>second</Text></View>;

You can use empty tags, I mean, <> and </>, whenever you just don't want any additional Container-Element (e.g. <View>), like below:

  render() {
    return (
      <>
        <Text>First</Text>

        <Text>Second</Text>
      </>
    );
  }

Example:

import React from 'react'
import { View, Text } from 'react-native'

import Reinput from 'reinput'

export default class ReinputWithHeader extends Reinput {
  constructor(props) {
    super(props);
  }
  render() {
    return (
      <>
        <View style={{backgroundColor: 'blue', flexDirection: 'row', alignSelf: 'stretch', height: 20}}>
          <Text>Blue Header</Text>
        </View>

        {super.render()}
      </>
    );
  }
}

Note: I tested and it works on react-native too; see also Fragments.

Preview: