Using Readonly vs. Immutability Libraries in TypeScript

Using Readonly vs. Immutability Libraries in TypeScript

Introduction

Immutable data patterns are widely recommended for safer, more predictable code, but TypeScript gives you multiple ways to express immutability: simple readonly modifiers, deep readonly utilities, or full-blown immutability libraries. Choosing the right approach affects developer ergonomics, runtime performance, interop with JavaScript libraries, and the type guarantees you actually get in your codebase.

In this deep tutorial for intermediate TypeScript developers, we'll define the problem space, compare native readonly features against popular immutability libraries, and provide actionable guidance to help you make an informed decision. You will learn how readonly behaves at compile time, how to model deep immutability with TypeScript utilities, and how to interoperate safely with libraries like Immer, Immutable.js, and immer-friendly patterns.

We also cover migration strategies, practical code examples, troubleshooting tips, and performance considerations. Along the way you'll find links to related TypeScript topics like configuring tsconfig, declaration files for JavaScript libraries, and strictness flags that influence type checking. By the end you should be able to adopt a consistent immutability strategy that balances runtime needs and type-safety across your project.

Background & Context

TypeScript's readonly modifier provides compile-time guarantees that a property won't be assigned to after initialization. However, readonly is shallow by default: nested objects remain mutable unless you explicitly model them. Immutability libraries provide runtime guarantees, helper APIs for producing new states, and sometimes structural sharing optimizations. Type-level utilities try to encode deep immutability in types, but those are often more complex and may not reflect runtime behavior unless paired with discipline or libraries.

Understanding how TypeScript's type system interacts with real-world JavaScript runtime is essential. Interop with third-party JS libraries often requires declaration files or using community-maintained types, and using strictness flags like noImplicitAny affects how confidently you can rely on typings. We'll touch on these practical matters when discussing adoption and migration.

Key Takeaways

• readonly in TypeScript is a useful compile-time tool but is shallow by default.
• Deep immutability requires either careful typing (mapped types / utility types) or runtime libraries (Immer, Immutable.js).
• Immutability libraries offer ergonomic APIs, runtime guarantees, and sometimes performance benefits like structural sharing.
• Choose based on project needs: small utility code may use readonly, complex state management benefits from Immer or persistent data structures.
• Interop with JS libraries needs proper declaration files or DefinitelyTyped packages; tsconfig strictness impacts detection of mistakes.
• Migration strategy and performance profiling are necessary before large-scale adoption.

Prerequisites & Setup

Before following the examples you'll need:

• Node.js and npm/yarn.
• TypeScript >= 4.x installed in your project (npm install --save-dev typescript).
• A basic tsconfig.json; consider reviewing options in Introduction to tsconfig.json: Configuring Your Project.
• Familiarity with mapped types and conditional types is helpful.

If your code uses external JavaScript libraries, confirm you have types via DefinitelyTyped or local declarations; see Using JavaScript Libraries in TypeScript Projects and Using DefinitelyTyped for External Library Declarations for guidance.

Main Tutorial Sections

1) Shallow readonly: what it guarantees (and what it doesn't)

TypeScript's readonly modifier prevents assignment to a property on a target type. Example:

type User = {
  readonly id: string
  name: string
}

const u: User = { id: '123', name: 'Alice' }
// u.id = '456' // Error: cannot assign to 'id'
u.name = 'Bob' // OK

This readonly guarantee is compile-time only and shallow: nested objects inside readonly properties are still mutable unless individually declared readonly. Use readonly for clear, low-cost invariants where you just need to avoid accidental reassignments to obvious fields.

2) Deep readonly with TypeScript utility types

To model deep immutability you can write or reuse a DeepReadonly mapped type:

type DeepReadonly<T> = T extends Function
  ? T
  : T extends Array<infer U>
  ? ReadonlyArray<DeepReadonly<U>>
  : T extends object
  ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
  : T

type Config = DeepReadonly<{ a: { b: number[] } }>

This produces type-level guarantees across nested structures, but it doesn't enforce immutability at runtime. You still can mutate if you coerce types or work around the type system. Deep types are most valuable for API contracts and preventing accidental writes, but they add complexity and can slow compilation in large projects.

3) Immutability libraries: runtime guarantees and ergonomics

Libraries like Immer and Immutable.js provide runtime immutability behavior along with convenient APIs.

• Immer lets you 'mutate' a draft and produces a new immutable state under the hood:

import produce from 'immer'

const base = { todos: [{ id: 1, text: 'Hi' }] }
const next = produce(base, draft => {
  draft.todos[0].text = 'Hello'
})

Immer codifies an ergonomic style and integrates well with Redux-like patterns. Immutable.js provides persistent data structures with structural sharing, which can reduce copying costs but uses its own APIs (Map, List) and requires mapping between plain objects and immutable structures.

Choosing a library depends on whether you want ergonomic mutable-style updates, persistent data structure performance, or pure runtime enforcement.

4) Interop: typing and declaration file considerations

When you add a JavaScript immutability library, ensure TypeScript knows the types. Use DefinitelyTyped packages where available or author local declarations. If types are missing you'll run into errors like "Cannot find name" or missing types at compile time — check Fixing the "Cannot find name 'X'" Error in TypeScript and Troubleshooting Missing or Incorrect Declaration Files in TypeScript.

If you author a small wrapper, consider writing a simple .d.ts; our guide on Writing a Simple Declaration File for a JS Module helps with that. Good typings make the difference between a smooth developer experience and constant friction.

5) Performance considerations: copies vs structural sharing

Immutability can be expensive if you naively deep-copy state on every change. Libraries mitigate this:

• Immer performs copy-on-write of changed branches only, producing shallow copies where necessary.
• Persistent data structures (Immutable.js, mori) share unchanged subtrees, improving memory use and potentially speed for some workloads.

Measure performance with realistic workloads before choosing. For most UI apps, Immer's draft mechanism is both ergonomic and fast. For high-throughput or memory-sensitive backends, persistent structures may be worth the cognitive and interop costs.

6) Mutability at runtime vs type-level immutability

Type-only immutability (readonly types) doesn't change runtime behavior. Example:

type R = { readonly x: { y: number } }

const r: R = { x: { y: 1 } }
// TS errors: r.x = { y: 2 }
(r.x as any).y = 2 // Allowed at runtime but bypasses types

If you need runtime guarantees (e.g., to avoid accidental mutation by other modules), pair types with runtime measures: Object.freeze for shallow freeze, deepFreeze functions for deep runtime freezing, or immutability libraries that enforce immutability by construction.

7) Working with arrays and collections

Arrays are mutable by default. TypeScript offers ReadonlyArray and readonly T[] types for shallow immutability. For deep immutability, you can use mapped types or libraries. Example with ReadonlyArray:

const nums: ReadonlyArray<number> = [1, 2, 3]
// nums.push(4) // Error
const doubled = nums.map(n => n * 2) // OK returns number[] unless you type it

When using libraries like Immer, you can treat arrays as mutable in the draft and Immer will handle producing the new array.

8) Gradual migration strategies

If you have an existing mutable codebase, migrating wholesale to immutability is risky. A practical plan:

1. Start by enabling stricter type checks (see Understanding strict Mode and Recommended Strictness Flags and Using noImplicitAny to Avoid Untyped Variables).
2. Introduce readonly on public APIs and DTOs.
3. Add deep-readonly types for critical interfaces.
4. Adopt an immutability library incrementally in new modules, or around complex reducers.
5. Write wrapper functions to convert between mutable inputs and immutable internals.

This reduces churn and isolates risk while providing measurable wins.

9) Tooling, debugging, and source maps

Immutability libraries can complicate debugging if they introduce proxies or wrapper types. Configure source maps and your build system correctly so stack traces and devtools remain helpful. See our guide on Generating Source Maps with TypeScript (sourceMap option) for build-time setup.

Also, consider logging or runtime checks. Object.freeze can help catch accidental writes in development but expect performance and compatibility trade-offs.

10) Module resolution and project layout considerations

When adopting libraries or adding declaration files, module resolution matters. If you encounter import headaches or long relative paths, use baseUrl and paths in tsconfig to simplify imports; see Controlling Module Resolution with baseUrl and paths. Keep declaration files near the modules they describe, and prefer community typings from DefinitelyTyped where possible to reduce maintenance.

Advanced Techniques

• Structural typing with branded immutability: use nominal-like brands to tag immutable values and prevent accidental mixing with mutable ones.

type Immutable<T> = T & { __immutable__: true }
function asImmutable<T>(x: T): Immutable<T> { return x as any }

This gives a lightweight guard at the type level for distinguishing immutable instances without deep copying.

• Hybrid approaches: store core data in a persistent structure (Immutable.js or Immer) and expose plain readonly objects for public APIs. Convert at boundaries to keep internal performance and external ergonomics.

• Memoization & structural sharing: pair persistent data structures with keyed selectors to optimize recalculation; libraries like reselect benefit from predictable identity changes.

• Frozen prototypes for runtime guarantees: use Object.freeze in development to fail-fast on accidental writes, then disable in production if needed.

Best Practices & Common Pitfalls

Dos:

• Use readonly for public API surface area to document immutability intent.
• Choose immutability libraries when you need runtime guarantees, ergonomic update patterns, or structural sharing.
• Add types for external libraries using DefinitelyTyped or local .d.ts files; see Using DefinitelyTyped for External Library Declarations and Writing a Simple Declaration File for a JS Module.
• Profile before and after changes; immutability sometimes increases memory churn if not using structural sharing.

Don'ts:

• Don't assume readonly prevents runtime mutation — it is a compile-time guard only.
• Avoid deep readonly everywhere in large projects without measuring compile-time impact.
• Don't mix immutable structures and mutable objects casually — prefer clear conversion functions at boundaries.

Troubleshooting tips:

• Errors about missing types? Check package types or add .d.ts; troubleshooting steps are covered in Troubleshooting Missing or Incorrect Declaration Files in TypeScript.
• If imports fail after moving files or adding aliases, review your tsconfig.json and module resolution strategy.

Real-World Applications

• UI state management: Immer is popular with React and Redux for writing immutable reducers in a familiar mutable style.
• Server-side caching: persistent data structures can help maintain history or undo/redo while sharing memory between versions.
• Multithreaded or concurrent systems: immutable data avoids locking and eliminates a class of bugs related to shared mutation.
• Public API design: return readonly views to callers to communicate intent and prevent accidental changes.

When integrating with third-party libraries, be intentional: some expect plain JS objects and won't work with Immutable.js structures without adapters. In those cases consider thin adapters or conversion layers, or prefer Immer which produces plain objects.

Conclusion & Next Steps

Choosing between TypeScript readonly and immutability libraries is a practical trade-off. readonly is simple, zero-runtime-cost, and great for public API contracts. Libraries like Immer and Immutable.js provide runtime guarantees, ergonomic update patterns, and performance optimizations that matter for complex state.

Next steps: experiment in a small feature branch, measure performance, and adopt stricter compiler flags via your tsconfig. For help with specific issues like missing types or declaration files, check related guides on declaration files and troubleshooting.

Enhanced FAQ

Q1: Isn't readonly enough for immutability? A1: readonly is a compile-time, shallow guarantee in TypeScript. It prevents reassignment of properties at the type level, but nested objects remain mutable unless also declared readonly. Also, types can be bypassed at runtime. If you need runtime guarantees or ergonomic immutable updates, consider an immutability library.

Q2: How do I type a deeply nested immutable object? A2: Use a DeepReadonly mapped type to recursively mark properties as readonly. Example provided above. Be mindful of performance and complexity: deep mapped types can slow down the TypeScript compiler in very large object graphs.

Q3: What library should I pick: Immer or Immutable.js? A3: Pick based on needs. Immer offers a low-friction API where you write code that looks mutable but produces immutable outputs; it returns plain JS objects, so interop is easy. Immutable.js offers persistent data structures with potential memory and performance benefits but requires using its API (Map, List) and conversions. For typical front-end state management, Immer is a good default.

Q4: How do I make sure third-party libraries have types? A4: Look for @types packages on npm (DefinitelyTyped). If none exist, you can author a small .d.ts file; see Writing a Simple Declaration File for a JS Module and the general guide on Troubleshooting Missing or Incorrect Declaration Files in TypeScript for strategies.

Q5: Does Object.freeze provide immutability guarantees? A5: Object.freeze provides shallow runtime immutability for objects and can be used to catch accidental writes in development. For deep freezing, you'd need to recursively freeze nested objects. Freezing has performance costs and doesn't help with structural sharing or ergonomic updates, so it is best used for small objects or in development mode.

Q6: How do strict compiler options affect working with immutability? A6: Enabling strict mode and flags like noImplicitAny improves the type coverage and surfaces mistakes earlier. They also make it easier to reason about immutability because you get better inference and fewer escaped anys. See Understanding strict Mode and Recommended Strictness Flags and Using noImplicitAny to Avoid Untyped Variables.

Q7: How should I handle arrays in immutable code? A7: Use ReadonlyArray or readonly T[] for shallow immutability at the type level. For ergonomic updates, Immer lets you mutate drafts of arrays while producing new arrays under the hood. Persistent collections like Immutable.js List provide structural sharing for arrays-like behavior.

Q8: Will immutability hurt performance? A8: Not necessarily. Naive deep copies can be slow. Immutability libraries often use copy-on-write or structural sharing to reduce costs. Profile realistic workloads and benchmark before fully committing. For UI apps, the ergonomic benefits often outweigh small overheads.

Q9: How do I gradually migrate a large codebase? A9: Start by adding readonly to public APIs, enable stricter compiler flags, add deep readonly types for critical modules, and introduce an immutability library incrementally. Isolate conversions at boundaries to minimize sweeping changes. See the migration steps in the Main Tutorial section above.

Q10: Any tips for debugging immutability issues? A10: Enable source maps, avoid opaque wrappers for debugging builds, and add runtime assertions when necessary. If you get unexpected mutations, use deepFreeze in development or add logging around state transitions. If types don't match runtime behavior, check declaration files or imports — guidance on module resolution and tsconfig can help, see Controlling Module Resolution with baseUrl and paths and Introduction to tsconfig.json: Configuring Your Project.