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.