# o1js Documentation

> Documentation for o1js, the TypeScript SDK for building zero-knowledge (zk) applications. Covers provable types, constraint systems, ZkProgram, smart contracts (zkApps) on Mina Protocol, recursive proofs, and deployment. Includes tutorials, API reference, and code examples.

Last updated: 2026-03-24

## Search

To find specific topics across the documentation, use the search endpoint:

* [Search](https://docs.o1labs.org/o1js/search?q=%s): Replace `%s` with your query, e.g. `?q=compile`, `?q=merkle+tree`, `?q=recursion`

## Documentation Versions

The "Current" docs track the latest o1js release. To confirm which version that is, check the top of the [Changelog](https://docs.o1labs.org/o1js/changelog).

Archived versions are available at versioned URLs:

* [Current (latest)](https://docs.o1labs.org/o1js/getting-started/intro): Default — all links in this file point here
* [v2.5.0](https://docs.o1labs.org/o1js/2.5.0/getting-started/intro): Archived docs for o1js 2.5.0
* [v2.4.0](https://docs.o1labs.org/o1js/2.4.0/getting-started/intro): Archived docs for o1js 2.4.0

To access any page for a specific version, insert the version number after `/o1js/` in the URL, e.g. `/o1js/2.5.0/basic-types/field`.

## Getting Started

* [Welcome](https://docs.o1labs.org/o1js/getting-started/intro): Overview of o1js — what it is, installation, and writing your first provable program
* [What is a ZK Constraint System?](https://docs.o1labs.org/o1js/getting-started/what-is-a-zk-constraint-system): How constraint systems work as arrays of gates, and how ZK programming differs from traditional programming

## Basic Types and Functions

* [Field](https://docs.o1labs.org/o1js/basic-types/field): The fundamental data type — field elements with modular arithmetic over a ~256-bit prime
* [Integers](https://docs.o1labs.org/o1js/basic-types/int): UInt8, UInt32, UInt64, and Int64 with built-in overflow and underflow checks
* [Keypairs and Signatures](https://docs.o1labs.org/o1js/basic-types/keypairs-and-signatures): Native Schnorr signatures, ECDSA for Ethereum interop, and foreign curve cryptography
* [Merkle Trees](https://docs.o1labs.org/o1js/basic-types/merkle-trees): IndexedMerkleMap, MerkleTree, and MerkleList for efficient data inclusion proofs
* [Arrays](https://docs.o1labs.org/o1js/basic-types/arrays): Fixed-size provable arrays using the Provable.Array helper
* [Structs](https://docs.o1labs.org/o1js/basic-types/structs): Creating complex provable types from configurations of Field-based properties
* [Hashing](https://docs.o1labs.org/o1js/basic-types/hashing): Poseidon (circuit-efficient), SHA-256, Keccak, and other hash functions for different use cases

## Writing Constraint Systems

* [Witnesses](https://docs.o1labs.org/o1js/writing-constraint-systems/witnesses): Prover-supplied values at runtime — blank spaces filled during proof generation
* [Conditional Logic](https://docs.o1labs.org/o1js/writing-constraint-systems/conditional-logic): How circuits handle control flow — all branches are always evaluated, use Provable.if() instead of if/else
* [Analyzing Constraints](https://docs.o1labs.org/o1js/writing-constraint-systems/analyzing-constraint-systems): Tools and techniques to inspect and minimize constraint counts for circuit optimization
* [ZkProgram](https://docs.o1labs.org/o1js/writing-constraint-systems/zk-program): Full proof flow — defining provable programs with methods, generating proofs, and verifying them

## ZkApps (Smart Contracts on Mina)

* [Introduction to zkApps](https://docs.o1labs.org/o1js/zkapps/intro): Smart contracts using off-chain execution with on-chain zk proof verification
* [SmartContract Basics](https://docs.o1labs.org/o1js/zkapps/smart-contracts): SmartContract class, on-chain state with @state decorator, transaction methods with @method decorator
* [Permissions](https://docs.o1labs.org/o1js/zkapps/permissions): 13 permission types controlling zkApp operations — editState, send, receive, setDelegate, and more
* [Events](https://docs.o1labs.org/o1js/zkapps/events): Emitting field arrays from methods, stored on archive nodes for off-chain consumption
* [Actions and Reducers](https://docs.o1labs.org/o1js/zkapps/actions-and-reducers): Dispatching actions with on-chain action state hash for batch state updates via reducers
* [Account Updates](https://docs.o1labs.org/o1js/zkapps/account-updates): The fundamental building blocks of zkApp transactions — how state changes are represented
* [Method Input Types](https://docs.o1labs.org/o1js/zkapps/provable-inputs): Using o1js provable types as zkApp method parameters while maintaining zero-knowledge properties
* [ZkProgram Proofs as Inputs](https://docs.o1labs.org/o1js/zkapps/zkprogram-proofs): Passing ZkProgram proofs into smart contract methods for proof composition patterns
* [Local Development](https://docs.o1labs.org/o1js/zkapps/local-development): LocalBlockchain for testing with pre-funded accounts and instant transaction finality
* [Deployment](https://docs.o1labs.org/o1js/zkapps/deployment): Deploying zkApps to Mina Devnet, Mainnet, and Zeko with key generation and fee configuration

## Advanced Concepts

* [Serialization](https://docs.o1labs.org/o1js/advanced-concepts/serialization): Serializing proofs and verification keys to and from base64 for storage and transport
* [Recursive ZK Proofs](https://docs.o1labs.org/o1js/advanced-concepts/recursion): Proof composition — proofs that verify other proofs for incrementally verifiable computation
* [Sideloaded Verification Keys](https://docs.o1labs.org/o1js/advanced-concepts/sideloaded-vks): Verifying arbitrary proofs at runtime by providing verification keys dynamically
* [On-Chain Preconditions](https://docs.o1labs.org/o1js/advanced-concepts/ZkApps/onChainPreconditions): Constraining on-chain values like account state, network timestamp, and balance in methods
* [Upgrading ZkApps](https://docs.o1labs.org/o1js/advanced-concepts/ZkApps/upgradability): Updating verification keys on deployed contracts to upgrade zkApp logic

## Tutorials

* [HMAC-SHA256 Basics](https://docs.o1labs.org/o1js/tutorials/hmac/hmac-basics): Understanding the HMAC-SHA256 algorithm and its cryptographic role
* [HMAC-SHA256 Implementation](https://docs.o1labs.org/o1js/tutorials/hmac/implementation): Building a complete HMAC-SHA256 zk circuit in o1js step by step
* [HMAC-SHA256 Tests](https://docs.o1labs.org/o1js/tutorials/hmac/tests): Validating the implementation against RFC 4231 test vectors
* [Next.js Integration](https://docs.o1labs.org/o1js/tutorials/frontent-integration-guides/next): Setting up a Next.js 15 project with o1js for frontend zkApp development
* [Angular Integration](https://docs.o1labs.org/o1js/tutorials/frontent-integration-guides/angular): Integrating Angular with Auro Wallet for zkApp development on Devnet

## API Reference

* [API Reference](https://docs.o1labs.org/o1js/api-reference/Introduction): Complete auto-generated TypeScript API documentation — classes, functions, types, namespaces, and variables for all o1js exports

## Changelog

* [Changelog](https://docs.o1labs.org/o1js/changelog): All notable changes, new features, deprecations, and bugfixes across o1js versions