javascript 37 lines · 9 steps

Building a text calendar in JavaScript

Turn a year and month into a grid of weeks, then render it as an aligned ASCII calendar.

Explained by highlit
Intermediate date-handling grid-layout modular-arithmetic string-formatting
1function generateCalendarGrid(year, month) {
2 const firstDay = new Date(year, month, 1);
3 const lastDay = new Date(year, month + 1, 0);
4 const daysInMonth = lastDay.getDate();
5 const startWeekday = firstDay.getDay();
6 
7 const weeks = [];
8 let week = new Array(7).fill(null);
9 
10 for (let day = 1; day <= daysInMonth; day++) {
11 const weekday = (startWeekday + day - 1) % 7;
12 week[weekday] = day;
13 if (weekday === 6 || day === daysInMonth) {
14 weeks.push(week);
15 week = new Array(7).fill(null);
16 }
17 }
18 
19 return weeks;
20}
21 
22function renderCalendar(year, month) {
23 const monthName = new Date(year, month).toLocaleString('default', { month: 'long' });
24 const headers = ['Su', 'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa'];
25 const weeks = generateCalendarGrid(year, month);
26 
27 const cell = (v) => (v === null ? ' ' : String(v).padStart(2, ' '));
28 const lines = [];
29 
30 lines.push(`${monthName} ${year}`.padStart(11 + Math.floor((monthName.length + 5) / 2)));
31 lines.push(headers.join(' '));
32 for (const week of weeks) {
33 lines.push(week.map(cell).join(' '));
34 }
35 
36 return lines.join('\n');
37}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Passing day 0 of the next month is a clean trick to get the last day of the current one.
  2. 2Modular arithmetic maps a running day count onto fixed weekday columns without branching.
  3. 3Separating grid generation from rendering keeps the date logic reusable and the output swappable.

Related explainers

javascript 
function initScrollSpy() {
  const links = Array.from(document.querySelectorAll('.nav a[href^="#"]'));
  const sections = links
    .map((link) => document.querySelector(link.getAttribute('href')))

Building a scroll spy with IntersectionObserver

intersectionobserver dom event-driven
Intermediate 7 steps
javascript 
const ROLE_PERMISSIONS = {
  admin: ['users:read', 'users:write', 'billing:read', 'billing:write'],
  manager: ['users:read', 'billing:read'],
  member: ['users:read'],

Role-based permissions middleware in Express

authorization middleware rbac
Intermediate 9 steps
javascript 
function attachThousandSeparators(input, { locale = 'en-US' } = {}) {
  const formatter = new Intl.NumberFormat(locale);
  const groupSep = formatter.format(11111).replace(/\d/g, '')[0] || ',';
  const decimalSep = formatter.format(1.1).replace(/\d/g, '')[0] || '.';

Live thousand separators without losing the caret

dom intl caret-preservation
Advanced 8 steps
javascript 
import { useReducer, useEffect } from "react";
 
const initialState = { status: "idle", data: null, error: null };
 

Building a data-fetching hook in React

custom-hooks usereducer data-fetching
Intermediate 9 steps
javascript 
const express = require('express');
const app = express();
 
app.get('/health', (req, res) => res.json({ status: 'ok' }));

Graceful shutdown in an Express server

graceful-shutdown signal-handling connection-tracking
Advanced 9 steps
javascript 
import { useState, useEffect, useCallback } from 'react';
 
function getColumnCount(width) {
  if (width < 640) return 1;

A responsive column hook in React

custom-hooks debouncing responsive-design
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Building a text calendar in JavaScript — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code