[SuperZen]

let calc = (function () {

  let value = 0;

  function setValue(val) {
    value = parseFloat(val.toFixed(2));
    return this
  }

  function plusVal(val) {
    value = parseFloat((value += val).toFixed(2));
    return this
  }

  function getValue() {
    return value
  }

  return {
    setValue,
    plusVal,
    getValue
  }

})();

console.log(calc.setValue(10).plusVal(30).getValue())

https://learn.javascript.ru/task/chain-calls