Answers for "react concatenate variable and string"

Concatenating variables and strings in React

href={"#demo" + this.state.id}

//You can also use ES6 string interpolation/template literals with 
//` (backticks) and ${expr} (interpolated expression),

href={`#demo${this.state.id}`}

source
https://stackoverflow.com/questions/39523040/concatenating-variables-and-strings-in-react

react concatenate string and component

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;
}