Code Explainers

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go 
package main
 
import (
	"errors"

Parsing and validating CLI flags in Go

cli-parsing validation error-handling
Intermediate 8 steps
ruby 
require "csv"
 
class SalesReport
  def initialize(path)

Aggregating CSV sales data in Ruby

data-aggregation memoization group_by
Intermediate 6 steps
javascript 
'use server'
 
import { revalidatePath } from 'next/cache'
import { redirect } from 'next/navigation'

How a Next.js Server Action updates a post

server-actions authorization validation
Intermediate 7 steps
java 
public class ThumbnailProcessor {
 
    private static final int MAX_CONCURRENCY = 4;
 

Bounded parallel thumbnail rendering in Java

concurrency thread-pool futures
Intermediate 7 steps
php 
<?php
 
namespace App\Support;
 

Locale-aware formatting with PHP's intl extension

internationalization encapsulation constructor-injection
Intermediate 7 steps
typescript 
type RetryOptions = {
  retries?: number;
  timeoutMs?: number;
  baseDelayMs?: number;

Retry with timeout and backoff in TypeScript

promises retry exponential-backoff
Intermediate 10 steps
rust 
use std::sync::{mpsc, Arc, Mutex};
use std::thread;
use std::time::Duration;
 

Building a thread pool in Rust

concurrency channels thread-pool
Advanced 9 steps
python 
from collections.abc import Mapping
from typing import Any, Iterator
 
 

Flattening nested config into dotted keys

recursion generators tree-traversal
Intermediate 7 steps
ruby 
module DurationFormatter
  UNITS = [
    ['week', 604_800],
    ['day', 86_400],

Turning seconds into human-readable durations in Ruby

greedy-decomposition modular-arithmetic formatting
Intermediate 7 steps
javascript 
const express = require('express');
 
const v1 = express.Router();
 

Versioning an API with Express Routers

api versioning routing modularity
Intermediate 10 steps
java 
public class SortedListMerger {
 
    public static int[] merge(int[] a, int[] b) {
        int[] result = new int[a.length + b.length];

Merging two sorted arrays in Java

two-pointers merging arrays
Beginner 6 steps
php 
<?php
 
namespace App\Support;
 

Merging query params onto a URL in PHP

url-parsing query-strings immutability
Intermediate 8 steps
typescript 
import { Pipe, PipeTransform, ChangeDetectorRef, NgZone, OnDestroy } from '@angular/core';
 
@Pipe({
  name: 'timeAgo',

A self-refreshing timeAgo pipe in Angular

impure-pipe change-detection timers
Advanced 10 steps
python 
import csv
import io
from datetime import datetime
 

Streaming a CSV export in Flask

streaming generators csv
Intermediate 9 steps
go 
package cache
 
import (
	"container/list"

Building a generic LRU cache in Go

lru-cache generics linked-list
Intermediate 8 steps
php 
<?php
 
class ImageUploadService
{

Validating file uploads safely in PHP

file-upload input-validation security
Intermediate 8 steps
ruby 
class Comment < ApplicationRecord
  belongs_to :post
  belongs_to :author, class_name: "User"
 

Live-updating comments with Turbo in Rails

turbo-streams callbacks associations
Intermediate 8 steps
python 
import time
from collections import defaultdict
from threading import Lock
 

Sliding-window login rate limiting in Flask

rate-limiting sliding-window thread-safety
Intermediate 7 steps