en.javascript.info/1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md

The solution is to return the object itself from every call.

let ladder = {
  step: 0,
  up() {
    this.step++;
*!*
    return this;
*/!*
  },
  down() {
    this.step--;
*!*
    return this;
*/!*
  },
  showStep() {
    alert( this.step );
*!*
    return this;
*/!*
  }
}

ladder.up().up().down().up().down().showStep(); // 1

We also can write a single call per line. For long chains it's more readable:

ladder
  .up()
  .up()
  .down()
  .up()
  .down()
  .showStep(); // 1