Cannot read properties of undefined (reading ‘DateTime’) even I check there is a data in React.js

I’m just getting learning React a few weeks ago. So, If I ask a pretty general question, I am sorry. But, It would be really appreciated if you help me out!

• Issue
  : Cannot read properties of undefined (reading ‘DateTime’)

• What I try to solve it
  : Firstly, I tried to solve it generally. I mean I add the condition to progress my code and re-render. Also, I check the data exist or not.

• Code

  const changeWeekendDateColor = () => {
    let eventArray = [];
    let weekendArray = [];
    if (weekendData && weekendData.event && weekendData.schedule) {
      let status = false;
      const dataEventLength = weekendData.event.length;
      const dataWeekendLength = weekendData.schedule.length;

      for (let i = 0; i <= dataEventLength; i++) {
        eventArray.push(weekendData.event[i].DateTime.split("T")[0]);
      }

      for (let i = 0; i <= dataWeekendLength; i++) {
        weekendArray.push(weekendData.schedule[i].split(" ")[0]);
      }
    }

    for (let i = 0; i <= eventArray.length; i++) {
      if (weekendArray.includes(eventArray[i])) return (status = false);
    }

    return status;
  };
  console.log(changeWeekendDateColor(weekendData));

WeekendData is this one

• What I expect
  : I add if statement

if (weekendData && weekendData.event && weekendData.schedule)

So, I think this should be work because if there is no data, it shouldn’t do the next line. I am not sure why my code couldn’t approach the weekendData.event.DateTime…