Manipulating Dates with Day.js — Set the Week of the Year and Month of a Date | by John Au-Yeung | Apr, 2024

Day.js is a JavaScript library that lets us manipulate dates in our apps.

In this article, we’ll look at how to use Day.js to manipulate dates in our JavaScript apps.

To set the week of the year in a Day.js date we can use the weekOfYear method available with the weekOfYear plugin:

const dayjs = require("dayjs");
const weekOfYear = require("dayjs/plugin/weekOfYear");
dayjs.extend(weekOfYear);const result = dayjs().week(1);
console.log(result);

We import the weekOfYear plugin with:

const weekOfYear = require("dayjs/plugin/weekOfYear");

And we set the week of the year to the first week by calling week with 1.

To set the ISO week of the year in a Day.js date we can use the isoWeek method available with the isoWeek plugin:

const dayjs = require("dayjs");
const isoWeek = require("dayjs/plugin/isoWeek");
dayjs.extend(isoWeek);const result = dayjs().isoWeek(1);
console.log(result);

We import the isoWeek plugin with:

const isoWeek = require("dayjs/plugin/isoWeek");

And we set the ISO week of the year to the first week by calling isoWeek with 1.

To set the month of Day.js date we can use the month method:

const dayjs = require("dayjs");
const result = dayjs().month(1);
console.log(result);

We call month with 1 to set the month the date to February. Month ranges from 0 for January to 11 for December.

Day.js is a JavaScript library that lets us manipulate dates in our apps.