Code Explainers

Browse the library

javascript 
function zip(keys, values) {
  if (keys.length !== values.length) {
    throw new RangeError('zip expects arrays of equal length');
  }

Three ways to zip arrays in JavaScript

arrays higher-order-functions pairing
Intermediate 6 steps
python 
from datetime import date, timedelta
from typing import Annotated
 
from fastapi import APIRouter, Depends, Query

Validating date ranges with FastAPI dependencies

dependency-injection validation pydantic
Intermediate 6 steps
java 
@RestController
@RequestMapping("/api/products")
@RequiredArgsConstructor
public class ProductBatchController {

Batch JSON Merge Patch in Spring

json-merge-patch rest-api partial-update
Intermediate 8 steps
typescript 
import {
  ArgumentsHost,
  Catch,
  ConflictException,

Turning TypeORM lock errors into 409s in NestJS

exception-handling optimistic-locking http-status
Intermediate 6 steps
python 
from collections.abc import MutableMapping
 
 
class CaseInsensitiveDict(MutableMapping):

Building a case-insensitive dict in Python

data structures abstract base classes dunder methods
Intermediate 8 steps
java 
public class TimedFetchService {
 
    private final ExecutorService executor = Executors.newFixedThreadPool(8);
    private final HttpClient httpClient = HttpClient.newHttpClient();

Enforcing HTTP timeouts with a Future

concurrency timeouts thread-pool
Intermediate 8 steps
php 
<?php
 
namespace App\Http\Requests\DataObjects;
 

Typed request DTOs in Laravel

data-transfer-object validation immutability
Intermediate 6 steps
typescript 
import { Component } from '@angular/core';
import { RouterLink, RouterLinkActive } from '@angular/router';
import { NgFor } from '@angular/common';
 

Building an active-route navbar in Angular

routing standalone-components accessibility
Intermediate 6 steps
ruby 
class OrderMailerPreview < ActionMailer::Preview
  def confirmation
    OrderMailer.confirmation(sample_order)
  end

Previewing Rails mailers with sample data

mailer-previews test-fixtures in-memory-objects
Beginner 6 steps
go 
package middleware
 
import (
	"net/http"

Role-based access control middleware in Gin

middleware authorization closures
Intermediate 7 steps
javascript 
import { useCallback, useEffect, useState } from 'react';
 
export function useLocalStorage(key, initialValue) {
  const readValue = useCallback(() => {

How a useLocalStorage hook syncs state in React

custom hooks localstorage state persistence
Intermediate 8 steps
python 
from fastapi import FastAPI, Request, status
from fastapi.encoders import jsonable_encoder
from fastapi.exceptions import RequestValidationError
from fastapi.responses import JSONResponse

Redacting sensitive fields in FastAPI errors

validation error-handling security
Intermediate 7 steps
java 
@Component
public class RefreshTokenSuccessHandler implements AuthenticationSuccessHandler {
 
    private final RefreshTokenService refreshTokenService;

Issuing JWT and refresh tokens on login in Spring

authentication jwt http-cookies
Intermediate 7 steps
php 
<?php
 
namespace App\Jobs;
 

Rendering invoice PDFs in a Laravel queue job

queued jobs pdf generation file storage
Intermediate 9 steps
typescript 
type Masker = (value: string) => string;
 
const maskEmail: Masker = (value) => {
  const [local, domain] = value.split("@");

Recursively masking sensitive data for logs

recursion regex data-masking
Intermediate 9 steps
ruby 
def render_table(rows)
  return "(no data)" if rows.empty?
 
  columns = rows.flat_map(&:keys).uniq

Rendering an ASCII table in Ruby

text-formatting column-alignment recursion
Intermediate 8 steps
go 
func UserDashboardCache(rdb *redis.Client, ttl time.Duration) gin.HandlerFunc {
	return func(c *gin.Context) {
		claims, ok := c.Get("claims")
		if !ok {

Per-user response caching in Gin with Redis

middleware caching redis
Advanced 9 steps
javascript 
import { useState } from 'react';
 
export function ReorderableList({ initialItems }) {
  const [items, setItems] = useState(initialItems);

Drag-to-reorder lists in React

drag-and-drop state-management immutable-updates
Intermediate 8 steps