Typing BigInt in TypeScript: Practical Guide for Intermediate Developers

Typing BigInt in TypeScript: Practical Guide for Intermediate Developers

Introduction

JavaScript's BigInt type lets you represent whole integers larger than 2^53 - 1, enabling accurate arithmetic for crypto, finance, and low-level system code. TypeScript exposes BigInt as a first-class type, but many nuances exist: literal types like 123n, runtime interop with Number, JSON serialization limits, and mixing BigInt with libraries that expect numbers. For intermediate developers, mastering BigInt typing means avoiding subtle runtime bugs, preserving type safety, and creating APIs that communicate intent clearly.

In this tutorial you'll learn how TypeScript models BigInt, how to write clear and safe APIs that accept and return bigint, strategies for converting to and from Number, guarding and narrowing runtime values, serializing BigInt in APIs, and patterns for interoperating with third-party libraries and Node.js APIs. We'll provide detailed code examples, step-by-step instructions, troubleshooting tips, and advanced techniques like literal bigint types, the satisfies operator for stable typed exports, and patterns for async code that rejects with typed errors when BigInt operations fail.

By the end you'll be able to make pragmatic choices about when to use bigint, how to type functions and classes around it, how to prevent accidental numeric loss, and how to handle serialization and external APIs safely and ergonomically.

Background & Context

BigInt addresses a long-standing JavaScript limitation: Number is a IEEE-754 double with safe integer range limited to 53 bits. BigInt provides arbitrary-precision integers and is useful for cryptography, financial ledgers, and indexing large datasets. TypeScript's type system includes the primitive type bigint and literal bigint types such as 9007199254740992n.

Typing BigInt interacts with many other areas of TypeScript programming. For example, when dealing with built-in objects you should know how BigInt interacts with Date and Math functions; read our guide on typing built-in objects for broader context. Also, when you design functions that may throw BigInt-related errors, consider typed error handling patterns from our typing error objects piece to keep exceptions predictable.

Understanding how BigInt flows through JSON APIs is critical because JSON does not support a bigint type; see best practices in our typing JSON payloads from external APIs guide for strategies on validation and transformation.

Key Takeaways

• BigInt is a primitive type in TypeScript: use the bigint annotation for function signatures and interfaces.
• Literal bigint types exist (e.g., 42n) and can be asserted using const assertions or validated with satisfies.
• BigInt and Number are not interoperable; explicit conversion is required.
• JSON cannot represent bigint; use string encoding or custom serializers and runtime guards.
• Use runtime type guards and TypeScript narrowing to safely work with unknown inputs.
• Follow best practices for error typing and promise rejection when working with BigInt operations.

Prerequisites & Setup

You should have a working TypeScript toolchain (TypeScript 4.0+ recommended; newer versions improve literal handling). Install Node.js (14+ recommended) and a code editor like VS Code. If you use bundlers or UIs, verify target compilation supports BigInt. In tsconfig.json, ensure your lib includes 'esnext' or 'es2020' so the global bigint type and BigIntConstructor are present.

Recommended sample tsconfig.json additions:

{
  "compilerOptions": {
    "target": "es2020",
    "lib": ["es2020", "dom"],
    "strict": true
  }
}

If you consume third-party libraries that accept large integers, check our guide on typing third-party libraries to design safe adapters and runtime guards.

Main Tutorial Sections

1. Primitive Types and Basic Usage

TypeScript exposes a primitive bigint type. Use bigint in annotations and accept bigint arguments when you require exact-integer arithmetic.

function addBig(a: bigint, b: bigint): bigint {
  return a + b
}

const x: bigint = 10n
const y = addBig(x, 20n)

Note that numeric literals for bigint end with n. The compiler infers literal types for such constants unless you use a non-literal assignment. Avoid mixing bigint with number; attempting to add bigint and number causes a type error.

2. Converting Between Number and BigInt

BigInt and Number are distinct. To convert explicitly use BigInt() and Number(). Be careful: converting a big bigint to number may lose precision.

const big = 9007199254740993n
const asNumber = Number(big) // 9007199254740992 — precision loss
const fromNumber = BigInt(42) // 42n

When converting from user input, validate range or prefer string-encoded bigint on the wire to avoid precision loss.

3. Literal bigint Types and const Assertions

Literal bigint types let you capture exact values in the type system. When exporting constants, prefer const assertions to keep literal types from widening.

export const DEFAULT_FEE = 50n as const
// DEFAULT_FEE has literal type 50n

For more on when to use const assertions and their trade-offs, see when to use const assertions.

4. Using the satisfies Operator for Stable Exports

The satisfies operator can help ensure exported configuration shapes remain typed while preserving literal bigint types where needed.

const fees = {
  transfer: 10n,
  withdraw: 20n
} satisfies Record<string, bigint>

export default fees

This keeps 'fees' typed as Record<string, bigint' while retaining literal values internally, which can be helpful for compile-time checks. Read more about satisfies in using the satisfies operator.

5. Type Guards and Narrowing for bigint

At runtime you must guard unknown values before treating them as bigint. Use typeof checks for primitives.

function isBigInt(value: unknown): value is bigint {
  return typeof value === 'bigint'
}

function printIfBig(value: unknown) {
  if (isBigInt(value)) {
    console.log(value + 1n)
  }
}

For complex shapes or library inputs, combine structural checks with numeric parsing. Our comparison of type assertions vs type guards vs narrowing is useful when choosing the right approach.

6. JSON Serialization and API Boundaries

JSON does not support bigint. Common patterns: encode bigint as string, use custom replacers, or compress to hex strings. On the server side, validate and reconstruct bigint values.

const payload = { id: '12345678901234567890', amount: '9007199254740993' }

// parse payload
const amount = BigInt(payload.amount)

When designing APIs, document that numeric fields are strings to prevent accidental Number parsing. See our typing JSON payloads from external APIs guide for validation strategies and runtime guards.

7. BigInt in Data Structures and Interfaces

Define interfaces that explicitly require bigint, and prefer wrappers for serialization.

interface LedgerEntry {
  id: string
  balance: bigint
}

function credit(entry: LedgerEntry, amount: bigint) {
  return { ...entry, balance: entry.balance + amount }
}

For collections sent to or from the server, provide converters: toWire and fromWire functions that handle string conversion and validation.

8. Working with Promises and Errors

BigInt operations may throw (for example, invalid BigInt parse). Ensure your promises reject with typed errors and document error shapes.

async function parseAmountFromApi(raw: string): Promise<bigint> {
  try {
    return BigInt(raw)
  } catch (err) {
    throw new TypeError('invalid amount')
  }
}

When typing async functions, our guide on typing promises that reject with specific error types is a good companion to design robust error handling.

9. Interoperability with Third-Party Libraries and Node

Many libraries expect number. Provide thin adapters that validate or convert values; never quietly coerce to Number when precision matters.

import { someLib } from 'legacy-lib'

function callLibWithSafeNumber(b: bigint) {
  const n = Number(b)
  if (BigInt(n) !== b) throw new Error('precision lost')
  return someLib(n)
}

For Node.js builtins that accept numbers (like buffer offsets), check how your project types Node.js APIs. See our article on typing Node.js built-in modules for guidance on safe adapters.

10. Debugging and Tooling

When debugging bigint issues, source maps and precise stack traces help. If transpilation or polyfills are involved, ensure BigInt usage maps correctly back to TS. For general source-map setup and async stack traces, consult debugging TypeScript code (source maps revisited).

Use runtime logs that display BigInt as strings when needed to avoid confusing console outputs in environments that do not display the 'n' suffix clearly.

Advanced Techniques

Once comfortable with basics, apply these expert patterns:

• Use wrapper types for API boundaries: define a SerializedBigInt type alias and implement centralized conversion utilities to avoid scattered BigInt parsing logic.
• Preserve literal types where they express intent (fees, limits) by combining const assertions and satisfies to enable narrow compile-time checks while keeping structural types generic.
• Build type-safe adapters for third-party libs where you must convert to number; include thorough runtime validation and unit tests to detect precision loss early. See patterns in our typing third-party libraries.
• For performance, prefer bigint operations that avoid unnecessary conversions. Converting between BigInt and Number in tight loops is costly; keep computations in one domain.
• When serializing, choose stable wire formats like decimal strings, hex strings, or fixed-size buffers and centralize serializers/deserializers to avoid mismatched formats.

Best Practices & Common Pitfalls

Dos:

• Do annotate public APIs with bigint when precision matters.
• Do validate all external input and treat JSON numeric fields as strings if they may exceed 2^53 - 1.
• Do centralize conversion logic and add tests that cover large integer edge cases.

Don'ts:

• Don't mix bigint and number directly; TypeScript won't implicitly convert and mixing at runtime can produce unreliable results.
• Don't assume JSON can carry bigint; naive JSON.parse will produce numbers and possible precision loss.
• Don't silently coerce big ints to numbers in libraries; prefer explicit adapters and clear errors.

Troubleshooting tips:

• If you get 'Cannot mix BigInt and other types' errors, search for operations mixing types and refactor to explicit conversion.
• Use precise unit tests that test values just above and below Number.MAX_SAFE_INTEGER to expose precision issues early.
• For unexpected runtime failures due to libraries, wrap calls and validate inputs and outputs; our typing third-party libraries guide shows patterns for building robust adapters.

Real-World Applications

• Finance and ledgers: storing account balances as bigint on the server prevents rounding and precision loss; serialize to decimal strings over JSON.
• Cryptography and hashing: BigInt is natural for representing large integers used in crypto algorithms.
• High-resolution counters and telemetry: when counters can exceed 2^53, use bigint to maintain correctness.

Example: a charge microservice can expose amounts as strings in its REST API. Internally, convert to bigint for arithmetic and persist as string or numeric database type that supports big integers. For more patterns around JSON and external data, reference our typing JSON payloads from external APIs.

Conclusion & Next Steps

BigInt brings powerful capabilities to TypeScript, but it requires intentional typing, conversion strategies, and careful serialization. Start by annotating APIs that need precise integers, centralize conversions, add runtime guards, and provide clear docs for downstream consumers. Next, explore related topics like typed error handling and advanced interop patterns to build reliable systems.

Recommended next reads: check our guides on typing error objects and typing third-party libraries to round out your safe BigInt usage patterns.

Enhanced FAQ

Q1: What is the difference between bigint and BigInt? A1: In TypeScript, 'bigint' is the primitive type for arbitrary-precision integers. 'BigInt' is a global constructor function (BigIntConstructor) you can call at runtime, e.g., BigInt('42') to produce a bigint. Use 'bigint' in type annotations and BigInt() for runtime conversion.

Q2: Can I use BigInt in JSON directly? A2: No. JSON does not have a bigint type. You must serialize bigints as strings (e.g., '9007199254740993') or encode them in a binary format. On the receiving end, parse the string into BigInt. Ensure API contracts explicitly describe this format. See serialization patterns in the JSON payload guide: typing JSON payloads from external APIs.

Q3: How do I guard unknown inputs before converting to bigint? A3: Use runtime type guards. For primitives, test typeof value === 'string' or 'bigint' and then attempt BigInt(value) inside a try/catch for parsing errors. For structured objects, validate fields with a schema validator or hand-written checks. Complement runtime guards with TypeScript's user-defined type guard functions to narrow types in code.

Q4: What about performance — is BigInt slower than Number? A4: BigInt operations are typically slower than native Number arithmetic, especially for very large values. Avoid unnecessary conversions between bigint and number in tight loops. Keep hot paths consistent in their numeric domain. Measure with your workload before optimizing prematurely.

Q5: How do I type functions that accept either number or bigint? A5: Prefer separate overloads or generic adapters. Example:

function addAny(a: number, b: number): number
function addAny(a: bigint, b: bigint): bigint
function addAny(a: any, b: any) { return a + b }

This approach ensures callers pass matching types. If you must accept both, implement runtime checks and convert to a common representation while documenting potential precision loss.

Q6: What happens when converting a big bigint to number? A6: Converting a bigint beyond Number.MAX_SAFE_INTEGER loses precision. Number(big) silently truncates to the nearest representable IEEE-754 value. Always validate before converting and throw or reject if precision would be lost.

Q7: How should I design APIs to be resilient to precision loss? A7: Use string-encoded integers over the wire, centralize serialization/deserialization, and document the wire format. Provide client libraries that hide serialization details and reconstruct bigint values safely. For public REST APIs, clearly document that numeric fields are strings representing whole integers.

Q8: How do I test BigInt logic effectively? A8: Write unit tests that target edge cases: values at Number.MAX_SAFE_INTEGER, values just above it, zero, negative values, and very large integers (many digits). Include tests that simulate corrupted wire formats to validate error handling. Add integration tests for adapters that convert to/from third-party libs.

Q9: Are there common security concerns with BigInt? A9: Ensure you validate lengths and formats for string-encoded bigints to avoid resource exhaustion (e.g., extremely long decimal strings). Don't accept unchecked user input to BigInt() without limits. For cryptographic contexts, use specialized libraries with vetted APIs.

Q10: Where can I learn more about combining BigInt with advanced TypeScript features? A10: Study const assertions and the satisfies operator to retain literal bigint types in constants, see when to use const assertions and using the satisfies operator. For patterns around typing async behavior and errors when parsing bigints, consult typing promises that reject with specific error types and typing error objects.

If you run into a specific problem in your code, share a minimal reproduction and the toolchain versions — I can help pinpoint the issue and propose a safe typing or runtime fix.