.weekday（）和.format（'d'）之间的moment.js的区别

Question

有人可以向我解释一下moment.js中.weekday（）和.format（'d'）的区别吗？

例如

console.log('month: ' + month.format('DD/MM/YYYY'));

let firstDay = moment(month).startOf('M');
console.log('/1 firstDay: ' + firstDay.format('DD/MM/YYYY'));
console.log('/1 firstDay.weekday(): ' + firstDay.weekday());
console.log('/1 firstDay.format("d"): ' + firstDay.format('d'));

firstDay = moment('01/08/2018', 'DD/MM/YYYY');
console.log('/2 firstDay: ' + firstDay.format('DD/MM/YYYY'));
console.log('/2 firstDay.weekday(): ' + firstDay.weekday());
console.log('/2 firstDay.format("d"): ' + firstDay.format('d'));

产生：

month: 09/08/2018
/1 firstDay: 01/08/2018
/1 firstDay.weekday(): 2
/1 firstDay.format("d"): 3
/2 firstDay: 01/08/2018
/2 firstDay.weekday(): 3
/2 firstDay.format("d"): 3