React Native getItemLayout

Question

I am implementing a SectionList showing a list of weeks which needs to scroll to the current week, thus I am using initialScrollIndex.

The problem I face is that I don't really understand what the index given to my _getItemLayout is? Sometimes data[index] === undefined, which doesn't make sense to me.

I need to know which section it is (data[?]) as each section contains another SectionList for the events that week, thus height isn't a constant.

_getItemLayout = (data, index) => {
  const rows = (data[index] === undefined) ? 1 : data[index].data.reduce((sum, value) => value.data.length + sum, 0);
  const height = (rows * 94);
  return {
    length: height,
    offset: height * index,
    index
  };
}

Answer 1

Its been long time, but for future readers thought to share some info.

There is a good article that explains the getItemLayout, please find it here

I also faced data[index] as undefined. The reason is that index is calculated considering section.data.length + 2 (1 for section header and 1 for section footer), you can find the code here (RN-52).

With SectionList we have to be very careful while processing index.