Code Explainers

Browse the library

python 
from django.db import transaction
from django.forms import inlineformset_factory
from django.shortcuts import get_object_or_404, redirect, render
 

Editing an order with inline formsets in Django

formsets transactions crud
Intermediate 9 steps
javascript 
function isValidCardNumber(input) {
  const digits = String(input).replace(/[\s-]/g, '');
 
  if (!/^\d{13,19}$/.test(digits)) {

Validating card numbers with Luhn

checksum validation luhn-algorithm
Intermediate 7 steps
python 
from flask import Blueprint, render_template, jsonify, request, abort
 
from .models import Article
 

Content negotiation in a Flask Blueprint

content-negotiation http-headers routing
Intermediate 7 steps
go 
package handlers
 
import (
	"net/http"

Sparse fieldsets in a Gin handler

sparse-fieldsets query-parsing json-serialization
Intermediate 9 steps
javascript 
import { useEffect, useState } from 'react';
 
export function useJobStatus(jobId, { interval = 3000 } = {}) {
  const [status, setStatus] = useState('pending');

Polling a job status with a React hook

custom-hooks polling cleanup
Intermediate 8 steps
typescript 
import { Directive, ElementRef, HostBinding, HostListener, Input, inject, signal } from '@angular/core';
 
@Directive({
  selector: '[appCopyToClipboard]',

A copy-to-clipboard directive in Angular

directives host bindings clipboard api
Intermediate 9 steps
rust 
use std::collections::BTreeSet;
use std::fs::File;
use std::io::{self, BufRead, BufReader, BufWriter, Write};
use std::path::Path;

Deduplicating and sorting words in Rust

file-io btreeset string-processing
Intermediate 6 steps
ruby 
module Api
  module Reports
    class DailyOrderTotalsController < Api::BaseController
      def index

Building a daily order totals report in Rails

sql aggregation json api query building
Intermediate 7 steps
python 
import os
import shutil
import tempfile
from contextlib import contextmanager

Transactional file operations in Python

transactions rollback closures
Intermediate 8 steps
php 
<?php
 
namespace App\Storage;
 

A race-free file counter in PHP

file-locking concurrency atomicity
Intermediate 7 steps
go 
package sse
 
import (
	"encoding/json"

Server-Sent Events streaming in Go

server-sent-events channels http-streaming
Intermediate 8 steps
javascript 
const express = require('express');
const { AsyncLocalStorage } = require('async_hooks');
const { randomUUID } = require('crypto');
 

Per-request logging context in Express

async-local-storage middleware structured-logging
Advanced 8 steps
typescript 
import slugify from "slugify";
 
interface SlugRepository {
  exists(slug: string): Promise<boolean>;

Generating unique URL slugs from titles

slugs normalization uniqueness
Intermediate 6 steps
rust 
pub fn levenshtein(a: &str, b: &str) -> usize {
    let a: Vec<char> = a.chars().collect();
    let b: Vec<char> = b.chars().collect();
 

Levenshtein distance with two rows in Rust

dynamic-programming edit-distance space-optimization
Intermediate 6 steps
java 
@Configuration
public class UserImportJobConfig {
 
    @Bean

How a Spring Batch CSV import job is wired

batch processing chunk oriented fault tolerance
Intermediate 10 steps
ruby 
module FixedWidthParser
  FIELDS = [
    { name: :record_type, range: 0...2 },
    { name: :account_id,  range: 2...12, type: :integer },

Parsing fixed-width records in Ruby

parsing data-driven-design type-coercion
Intermediate 5 steps
python 
from django.core.paginator import Paginator
from django.shortcuts import render
 
from .models import Product

Filtering and paginating a Django product list

queryset pagination filtering
Intermediate 7 steps
go 
package handlers
 
import (
	"encoding/json"

Parsing search query params in a Go handler

http-handlers input-validation defaults
Intermediate 7 steps