Code Explainers

Browse the library

php 
<?php
 
final class RememberMeCookie
{

Signed remember-me cookies in PHP

authentication hmac cookies
Intermediate 8 steps
typescript 
type Middleware<TIn, TOut> = (ctx: TIn) => Promise<TOut> | TOut;
 
class Pipeline<TIn, TOut> {
  private constructor(private readonly run: Middleware<TIn, TOut>) {}

A type-safe async middleware pipeline

generics type-safety middleware
Advanced 9 steps
rust 
use std::time::{Duration, Instant};
 
pub struct Ewma {
    alpha: f64,

A time-decayed moving average in Rust

exponential-smoothing time-decay state-machine
Intermediate 8 steps
java 
@Component
public class HeaderMergeFilter {
 
    private static final String PER_REQUEST_HEADERS = HeaderMergeFilter.class.getName() + ".headers";

Merging default and per-request headers in Spring

webclient filters http-headers
Intermediate 8 steps
ruby 
class Spellchecker
  ALPHABET = ('a'..'z').to_a.freeze
 
  def initialize(dictionary)

Building a Norvig-style spellchecker in Ruby

edit-distance levenshtein dynamic-programming
Advanced 10 steps
go 
package middleware
 
import (
	"net/http"

Wrapping Gin requests in a DB transaction

middleware transactions error-handling
Intermediate 8 steps
javascript 
import { NextResponse } from 'next/server';
import { db } from '@/lib/db';
 
function csvCell(value) {

Streaming a CSV export in a Next.js route

streaming csv readablestream
Advanced 9 steps
python 
from flask import Flask, request, g, jsonify
from flask_babel import Babel, gettext as _, format_datetime
from datetime import datetime
 

Per-request localization in Flask with Babel

i18n content-negotiation request-lifecycle
Intermediate 8 steps
php 
<?php
 
namespace App\Http\Middleware;
 

Caching HTTP responses with Laravel middleware

middleware caching http
Intermediate 8 steps
typescript 
type Countdown = {
  days: number;
  hours: number;
  minutes: number;

Building a self-stopping countdown timer

date-math closures timers
Intermediate 9 steps
rust 
use std::fs::File;
use std::io::{Read, Seek, SeekFrom};
 
#[derive(Debug)]

Parsing an HTTP Range header in Rust

http-range parsing file-io
Intermediate 10 steps
java 
import java.util.HashSet;
import java.util.Set;
 
public final class CollectionDiff<T> {

Diffing two collections with set operations

set-operations immutability generics
Intermediate 8 steps
ruby 
class Order < ApplicationRecord
  belongs_to :customer
  has_many :line_items
  has_many :products, through: :line_items

How scopes compose in a Rails model

scopes activerecord subqueries
Intermediate 7 steps
go 
package handlers
 
import (
	"encoding/base64"

Cursor pagination in a Gin handler

pagination cursor rest-api
Intermediate 8 steps
javascript 
class MovingAverage {
  constructor(windowSize) {
    if (!Number.isInteger(windowSize) || windowSize <= 0) {
      throw new RangeError('windowSize must be a positive integer');

A rolling average over a fixed window

circular-buffer streaming async-generators
Intermediate 7 steps
python 
from django.core.cache import cache
from django.core.cache.utils import make_template_fragment_key
from django.db.models.signals import post_save, post_delete
from django.dispatch import receiver

Busting template fragment caches in Django

caching signals cache-invalidation
Intermediate 4 steps
go 
package metrics
 
import (
	"sync"

A thread-safe sliding-window average in Go

concurrency sliding-window running-average
Intermediate 8 steps
java 
public record AppConfig(
        String host,
        int port,
        String databaseUrl,

Loading typed config from env vars in Java

records configuration environment-variables
Intermediate 6 steps