Chaining

There’s a ladder object that allows you to go up and down:

          let ladder = {
  step: 0,
  up() {
    this.step++;
  },
  down() {
    this.step--;
  },
  showStep: function() { // shows the current step
    alert( this.step );
  }
};
        

Now, if we need to make several calls in sequence, we can do it like this:

          ladder.up();
ladder.up();
ladder.down();
ladder.showStep(); // 1
ladder.down();
ladder.showStep(); // 0
        

Modify the code of up, down, and showStep to make the calls chainable, like this:

ladder.up().up().down().showStep().down().showStep(); // shows 1 then 0

Such an approach is widely used across JavaScript libraries.

Open a sandbox with tests.

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().showStep().down().showStep(); // shows 1 then 0

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

          ladder
  .up()
  .up()
  .down()
  .showStep() // 1
  .down()
  .showStep(); // 0
        

Open the solution with tests in a sandbox.