Code Explainers

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rust 
use std::collections::HashMap;
 
#[derive(Debug, Clone)]
struct Order {

Aggregating Rust data with fold and entry

fold hashmap ownership
Intermediate 6 steps
go 
func (h *TransactionHandler) ExportCSV(c *gin.Context) {
	ctx := c.Request.Context()
	filters := parseTransactionFilters(c)
 

Streaming a CSV export in Gin

streaming csv-export database-cursor
Intermediate 8 steps
php 
<?php
 
namespace App\Rules;
 

How a custom phone validation rule works in Laravel

validation custom-rules dependency
Intermediate 6 steps
ruby 
class Comment < ApplicationRecord
  belongs_to :commentable, polymorphic: true, counter_cache: true
  belongs_to :author, class_name: "User"
 

How polymorphic comments work in Rails

polymorphic-association concerns counter-cache
Intermediate 8 steps
python 
import json
import logging
 
import stripe

Handling Stripe webhooks in Django

webhooks signature-verification idempotency
Intermediate 7 steps
javascript 
import { useState, useEffect, useCallback, useRef } from "react";
 
export function usePersistentForm(storageKey, initialValues) {
  const [values, setValues] = useState(() => {

A React hook that persists form state to localStorage

custom-hooks localstorage debounce
Intermediate 9 steps
rust 
use axum::{extract::State, http::StatusCode, response::IntoResponse, Json};
use serde::{Deserialize, Serialize};
use uuid::Uuid;
 

Building a typed create_user handler in Axum

serde extractors sqlx
Intermediate 7 steps
java 
@Component
public class JwtAuthenticationFilter extends OncePerRequestFilter {
 
    private final JwtTokenProvider tokenProvider;

How a JWT auth filter works in Spring

authentication jwt servlet-filter
Intermediate 8 steps
typescript 
function throttle<T extends (...args: any[]) => void>(
  fn: T,
  limit: number
): (...args: Parameters<T>) => void {

Building a trailing-edge throttle in TypeScript

throttling closures generics
Intermediate 7 steps
go 
package store
 
import (
	"database/sql"

Wrapping and inspecting errors in Go

error-handling error-wrapping sentinel-errors
Intermediate 8 steps
php 
<?php
 
namespace App\Support;
 

Building a URL slug from any title in PHP

regex transliteration string-processing
Intermediate 8 steps
python 
from collections import deque
 
 
class RollingAverage:

A rolling average over a fixed window

sliding-window running-sum deque
Intermediate 7 steps
javascript 
function groupBy(items, keySelector) {
  const resolveKey = typeof keySelector === 'function'
    ? keySelector
    : (item) => item[keySelector];

Building a flexible groupBy in JavaScript

higher-order-functions reduce data-transformation
Intermediate 6 steps
rust 
use axum::{
    response::sse::{Event, KeepAlive, Sse},
    routing::get,
    Router,

Streaming live price ticks with SSE in Axum

server-sent-events broadcast-channel streaming
Advanced 8 steps
java 
@Entity
@Table(name = "accounts")
public class Account {
 

Optimistic locking with @Version in Spring

optimistic-locking jpa concurrency
Advanced 7 steps
typescript 
interface TokenBucketOptions {
  capacity: number;
  refillPerSecond: number;
}

How a token bucket rate limiter works

rate-limiting token-bucket lazy-evaluation
Intermediate 7 steps
go 
package main
 
import (
	"context"

Graceful HTTP shutdown in Go

graceful-shutdown signals goroutines
Intermediate 8 steps
php 
<?php
 
declare(strict_types=1);
 

Streaming file lines with a PHP generator

generators lazy-evaluation file-io
Intermediate 7 steps