Practical Data Transformations

This skill covers the data transformations you do every day: working with arrays, reshaping objects, normalizing API responses, grouping data, and safely accessing nested values. Each section shows the imperative approach first, then the functional equivalent, with honest assessments of when each approach shines.

Detailed Guide

Read the detailed guide before executing this skill. It retains the complete procedure and reference material. Treat its safety, prerequisites, and validation requirements as mandatory. For focused work, load the relevant sections; for end-to-end work, read the guide completely.

When to Use

You need to transform arrays, objects, grouped data, or nested values in TypeScript.
The task involves reshaping API responses, null-safe access, aggregation, or normalization.
You want practical functional patterns for everyday data work instead of low-level loops.

6. Real-World Examples

Example 1: Transform API Response to UI-Ready Data

// API response
interface ApiOrder {
  order_id: string;
  customer: {
    id: string;
    full_name: string;
  };
  line_items: Array<{
    product_id: string;
    product_name: string;
    qty: number;
    unit_price: number;
  }>;
  order_date: string;
  status: 'pending' | 'processing' | 'shipped' | 'delivered';
}

// What the UI needs
interface OrderSummary {
  id: string;
  customerName: string;
  itemCount: number;
  total: number;
  formattedTotal: string;
  date: string;
  statusLabel: string;
  statusColor: string;
}

// Transformation
const STATUS_CONFIG: Record<string, { label: string; color: string }> = {
  pending: { label: 'Pending', color: 'yellow' },
  processing: { label: 'Processing', color: 'blue' },
  shipped: { label: 'Shipped', color: 'purple' },
  delivered: { label: 'Delivered', color: 'green' },
};

const formatCurrency = (cents: number): string =>
  `$${(cents / 100).toFixed(2)}`;

const formatDate = (iso: string): string =>
  new Date(iso).toLocaleDateString('en-US', {
    month: 'short',
    day: 'numeric',
    year: 'numeric',
  });

const toOrderSummary = (order: ApiOrder): OrderSummary => {
  const total = order.line_items.reduce(
    (sum, item) => sum + item.qty * item.unit_price,
    0
  );

  const status = STATUS_CONFIG[order.status] ?? STATUS_CONFIG.pending;

  return {
    id: order.order_id,
    customerName: order.customer.full_name,
    itemCount: order.line_items.reduce((sum, item) => sum + item.qty, 0),
    total,
    formattedTotal: formatCurrency(total),
    date: formatDate(order.order_date),
    statusLabel: status.label,
    statusColor: status.color,
  };
};

// Transform all orders
const toOrderSummaries = (orders: ApiOrder[]): OrderSummary[] =>
  orders.map(toOrderSummary);

Example 2: Merge User Settings with Defaults

interface AppSettings {
  theme: {
    mode: 'light' | 'dark' | 'system';
    primaryColor: string;
    fontSize: 'small' | 'medium' | 'large';
  };
  notifications: {
    email: boolean;
    push: boolean;
    sms: boolean;
    frequency: 'immediate' | 'daily' | 'weekly';
  };
  privacy: {
    showProfile: boolean;
    showActivity: boolean;
    allowAnalytics: boolean;
  };
}

type DeepPartial<T> = {
  [P in keyof T]?: T[P] extends object ? DeepPartial<T[P]> : T[P];
};

const DEFAULT_SETTINGS: AppSettings = {
  theme: {
    mode: 'system',
    primaryColor: '#007bff',
    fontSize: 'medium',
  },
  notifications: {
    email: true,
    push: true,
    sms: false,
    frequency: 'immediate',
  },
  privacy: {
    showProfile: true,
    showActivity: true,
    allowAnalytics: true,
  },
};

const deepMergeSettings = (
  defaults: AppSettings,
  user: DeepPartial<AppSettings>
): AppSettings => ({
  theme: { ...defaults.theme, ...user.theme },
  notifications: { ...defaults.notifications, ...user.notifications },
  privacy: { ...defaults.privacy, ...user.privacy },
});

// Usage
const userPreferences: DeepPartial<AppSettings> = {
  theme: { mode: 'dark' },
  notifications: { sms: true, frequency: 'daily' },
};

const finalSettings = deepMergeSettings(DEFAULT_SETTINGS, userPreferences);

Example 3: Group Orders by Customer with Totals

interface Order {
  id: string;
  customerId: string;
  customerName: string;
  items: Array<{ name: string; price: number; quantity: number }>;
  date: string;
}

interface CustomerOrderSummary {
  customerId: string;
  customerName: string;
  orderCount: number;
  totalSpent: number;
  orders: Order[];
}

const calculateOrderTotal = (order: Order): number =>
  order.items.reduce((sum, item) => sum + item.price * item.quantity, 0);

const groupOrdersByCustomer = (orders: Order[]): CustomerOrderSummary[] => {
  const grouped = groupBy((order: Order) => order.customerId)(orders);

  return Object.entries(grouped).map(([customerId, customerOrders]) => ({
    customerId,
    customerName: customerOrders[0].customerName,
    orderCount: customerOrders.length,
    totalSpent: customerOrders.reduce(
      (sum, order) => sum + calculateOrderTotal(order),
      0
    ),
    orders: customerOrders,
  }));
};

Example 4: Safely Access Deeply Nested Config

interface AppConfig {
  services?: {
    api?: {
      endpoints?: {
        users?: string;
        orders?: string;
        products?: string;
      };
      auth?: {
        type?: 'bearer' | 'basic' | 'oauth';
        token?: string;
      };
    };
    database?: {
      primary?: {
        host?: string;
        port?: number;
        name?: string;
      };
    };
  };
}

import * as O from 'fp-ts/Option';
import { pipe } from 'fp-ts/function';

// Create a type-safe config accessor
const getConfigValue = <T>(
  config: AppConfig,
  path: (config: AppConfig) => T | undefined,
  defaultValue: T
): T => path(config) ?? defaultValue;

// Usage with optional chaining (simplest)
const apiUsersEndpoint = getConfigValue(
  config,
  c => c.services?.api?.endpoints?.users,
  '/api/users'
);

// For more complex scenarios, use Option
const getEndpoint = (config: AppConfig, name: 'users' | 'orders' | 'products'): string =>
  pipe(
    O.fromNullable(config.services),
    O.flatMap(s => O.fromNullable(s.api)),
    O.flatMap(a => O.fromNullable(a.endpoints)),
    O.flatMap(e => O.fromNullable(e[name])),
    O.getOrElse(() => `/api/${name}`)
  );

// Reusable pattern for multiple values
const getDbConfig = (config: AppConfig) => ({
  host: config.services?.database?.primary?.host ?? 'localhost',
  port: config.services?.database?.primary?.port ?? 5432,
  name: config.services?.database?.primary?.name ?? 'app',
});

7. When to Use What

Use Native Methods When:

Simple transformations: .map(), .filter(), .reduce() are perfectly good
No composition needed: You're doing a one-off transformation
Team familiarity: Everyone knows native methods
Optional chaining suffices: obj?.prop?.value ?? default handles your null-safety needs

// Native is fine here
const activeUserNames = users
  .filter(u => u.isActive)
  .map(u => u.name);

Use fp-ts When:

Chaining operations that might fail: Multiple steps where each can return nothing
Composing transformations: Building reusable transformation pipelines
Type-safe error handling: You want the compiler to track potential failures
Complex data pipelines: Many steps that benefit from explicit composition

// fp-ts shines here
const result = pipe(
  users,
  A.findFirst(u => u.id === userId),
  O.flatMap(u => O.fromNullable(u.profile)),
  O.flatMap(p => O.fromNullable(p.settings)),
  O.map(s => s.theme),
  O.getOrElse(() => 'default')
);

Use Custom Utilities When:

Domain-specific operations: groupBy, countBy, sumBy for your data
Repeated patterns: You find yourself writing the same transformation many times
Team conventions: Establishing consistent patterns across the codebase

// Custom utility pays off when used repeatedly
const revenueByRegion = sumBy(
  (sale: Sale) => sale.region,
  (sale: Sale) => sale.amount
)(sales);

Performance Considerations

Chaining creates intermediate arrays: arr.filter().map() creates one array, then another
For hot paths, consider reduce: One pass through the data
Measure before optimizing: The readability cost of optimization is often not worth it

// If performance matters (and you've measured!)
const result = items.reduce((acc, item) => {
  if (item.isActive) {
    acc.push(item.name.toUpperCase());
  }
  return acc;
}, [] as string[]);

// vs the more readable (but 2-pass) version
const result = items
  .filter(item => item.isActive)
  .map(item => item.name.toUpperCase());

Limitations

Use this skill only when the task clearly matches the scope described above.
Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

fp-data-transforms