spreadSheet.js

const Values = function() {
  this.values = {};
}

Values.prototype.get_value = function(variable) {
  return this.values[variable];
}

Values.prototype.set_value = function(variable, value) {
  let newValue = 0;
  if (value[0] == '=') {
    value = value.slice(1);
    value = value.split('+');
    for (let i = 0; i < value.length; i++) {
      newValue += parseInt(this.get_value(value[i]));
    }
    this.values[variable] = newValue;
  } else {
    this.values[variable] = value;
  }
}

const newValues = new Values();
newValues.set_value('A1', '1');
newValues.set_value('B1', '2');
newValues.set_value('D1', '24');
newValues.set_value('C1', '=A1+B1+D1');
newValues.get_value('A1');
newValues.get_value('C1');