What happens to the JS class object?

Question

JavaScript

Good day to all!
There was such a serious question, I am doing a chain-chain in JS. From the first "add" method I return this, in the second I mutate the state of the objects field of this.
And so for some reason this field mutates even before the second method has been called.
I may be stupid somewhere, but I’ve already specifically driven, MB someone will help :\

add(data) {
    switch(data.kind) {
      case 'Placemark':
        this.object = placemark(data)
        break
    }

    console.log(this)
    console.log('in add')
    return this
  }

  setInto(clasterName) {
    console.log(this)
    console.log('i am here')
    if(! this.objects[clasterName]) {
      this.objects[clasterName] = []
    }
    this.objects[clasterName].push(this.object)
  }

These are the same two methods7
First, the mutated instance is displayed in the console, then "in add", confirming that we are still in the first method. Already then the logs are displayed from the second method. What is "magic"?

Call:

map.add({kind: 'Placemark'}).setInto('hrenSobachiy')