#reduce code explainers

javascript

const DISCOUNT_CODES = {
  SAVE10: { type: 'percent', value: 0.1 },
  WELCOME5: { type: 'fixed', value: 5 },
  FREESHIP: { type: 'shipping', value: Infinity },

How a cart total is calculated with discounts

reduce lookup-table options-object

Intermediate 8 steps

typescript

type Flatten = Record<string, unknown>;
 
function isPlainObject(value: unknown): value is Record<string, unknown> {
  return (

Flattening nested objects into dotted keys

recursion reduce type-guards

Intermediate 7 steps

ruby

class TemplateInterpolator
  PLACEHOLDER = /\{\{\s*([\w.]+)\s*\}\}/
 
  def initialize(strict: false)

Interpolating templates with dotted keys in Ruby

regex string-interpolation hash-traversal

Intermediate 6 steps

ruby

class Pipeline
  def initialize
    @middlewares = []
  end

Building a middleware pipeline in Ruby

closures middleware composition

Advanced 8 steps

python

from collections import ChainMap
from functools import reduce
from typing import Any, Iterable, Mapping

Five ways to merge dicts in Python

dictionaries merging recursion

Intermediate 8 steps

typescript

interface Order {
  id: number;
  customerId: string;
  total: number;

A type-safe groupBy in TypeScript

generics reduce type-inference

Intermediate 6 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

javascript

function flatten(arr) {
  const result = [];
  for (const item of arr) {
    if (Array.isArray(item)) {

Flattening nested arrays with recursion

recursion arrays reduce

Intermediate 7 steps