Code Explainers

Browse the library

ruby 
class Membership < ApplicationRecord
  belongs_to :organization
  belongs_to :user
 

Enforcing scoped uniqueness in Rails

validations uniqueness callbacks
Intermediate 8 steps
typescript 
import { Component, HostBinding, Input } from '@angular/core';
 
type ProgressVariant = 'success' | 'warning' | 'danger';
 

A CSS-driven progress ring in Angular

host-bindings css-custom-properties input-setters
Intermediate 8 steps
python 
import heapq
from collections import Counter
from typing import Iterable, Hashable
 

Finding the top-N items in a stream

heaps counting generators
Intermediate 5 steps
php 
<?php
 
namespace App\Providers;
 

Defining authorization gates in Laravel

authorization gates service-provider
Intermediate 6 steps
javascript 
import { NextResponse } from 'next/server';
import { Redis } from '@upstash/redis';
 
const redis = Redis.fromEnv();

Sliding-window rate limiting in a Next.js route

rate-limiting redis sorted-set
Advanced 8 steps
python 
from flask import Blueprint, jsonify
from sqlalchemy import text
from sqlalchemy.exc import SQLAlchemyError
 

Building a health check endpoint in Flask

health-check blueprint error-handling
Intermediate 6 steps
java 
@Component
public class RegionCacheWarmer implements SmartInitializingSingleton {
 
    private static final Logger log = LoggerFactory.getLogger(RegionCacheWarmer.class);

Warming a Spring cache at startup

caching startup-hook dependency-injection
Intermediate 7 steps
typescript 
import { Controller, Param, Sse, MessageEvent } from '@nestjs/common';
import { Observable, interval, merge } from 'rxjs';
import { filter, map, takeWhile } from 'rxjs/operators';
import { JobService } from './job.service';

Streaming job progress with SSE in NestJS

server-sent-events reactive-streams rxjs
Intermediate 8 steps
go 
package breaker
 
import (
	"errors"

How a circuit breaker works in Go

circuit-breaker state-machine concurrency
Intermediate 8 steps
php 
<?php
 
final class TaskProgressReporter
{

Tracking task progress in a JSON file

state-persistence atomic-writes json
Intermediate 9 steps
ruby 
class WebhookSignatureConstraint
  def initialize(provider)
    @provider = provider
  end

Verifying webhook signatures with Rails routing constraints

routing constraints hmac webhooks
Advanced 7 steps
rust 
#[derive(Debug, Clone, PartialEq)]
pub enum Token {
    Number(f64),
    Plus,

How a tokenizer turns text into tokens

lexing enums iterators
Intermediate 8 steps
javascript 
const MAX_FILE_SIZE = 5 * 1024 * 1024;
 
const ALLOWED_TYPES = {
  'image/jpeg': ['jpg', 'jpeg'],

Validating file uploads by content, not just claims

input-validation security magic-bytes
Intermediate 7 steps
python 
import pandas as pd
import numpy as np
 
 

Cleaning a customer DataFrame with pandas

data-cleaning regex normalization
Intermediate 9 steps
java 
public final class EmailNormalizer {
 
    private static final Pattern EMAIL_PATTERN = Pattern.compile(
        "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$"

Normalizing email addresses in Java

validation regex normalization
Intermediate 8 steps
typescript 
import {
  Controller,
  All,
  Req,

Catch-all routes and error shaping in NestJS

exception-handling routing middleware
Intermediate 6 steps
ruby 
module UniqueJob
  extend ActiveSupport::Concern
 
  class_methods do

Deduplicating Active Job enqueues in Rails

concurrency idempotency caching
Advanced 9 steps
rust 
use axum::{
    extract::{FromRequestParts, Host},
    http::{request::Parts, StatusCode},
};

Multi-tenant DB routing in Axum extractors

multi-tenancy extractors connection-pooling
Advanced 8 steps