test.es

// Github recognises this as Erlang
let x = 1;
export default class Demo {
  constructor( ...args ) {
    this.arr = [ ...args ];
  }
}

test.es2015

// Github recognises this as plain text
let x = 1;
export default class Demo {
  constructor( ...args ) {
    this.arr = [ ...args ];
  }
}

test.es6

// Github recognises this as ES2015
let x = 1;
export default class Demo {
  constructor( ...args ) {
    this.arr = [ ...args ];
  }
}