Clarification for the use of eachWeekOfInterval

[sfabriece]

The following is giving me an unexpected answer:

const { eachWeekOfInterval, parseISO } = require("date-fns");

const start = parseISO('2021-03-29T14:42:44.572Z');
const end = parseISO('2021-04-04T14:42:44.572Z');

console.log(eachWeekOfInterval({start, end}, {startsOn: 4}));

//0: Thu Mar 25 2021 00:00:00 GMT+0100 (Central European Standard Time)
//1: Thu Apr 01 2021 00:00:00 GMT+0200 (Central European Summer Time)

I expected to receive only 1 entry for Thursday April 1 2021.

[fturmel]

Hi @sfabriece,

The way I understand it, by using { weekStartsOn: 4 } you are defining Thursdays as the first day of the week. The default value for this option is 0 (Sunday).

March 29th is a Monday, and is part of the week that starts on Thursday March 25th.
April 4th is a Sunday, and is part of the week that starts on Thursday April 1st.

Hope that explains it!