Abi Fundamentals - Voltaire

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Run ABI examples in the interactive playground

Conceptual Guide - For API reference and method documentation, see ABI API.

The Application Binary Interface (ABI) is Ethereum’s standard for encoding function calls, return values, events, and errors when interacting with smart contracts. This guide teaches ABI fundamentals using Tevm.

What is ABI?

ABI defines how to encode and decode data for contract interactions:

Function calls - Encode parameters into calldata
Function returns - Decode return values from execution
Events - Encode/decode log entries
Errors - Encode custom error data

Without ABI, contracts couldn’t communicate. It’s the protocol that lets you call transfer(address,uint256) or parse Transfer events.

Function Selectors

Every function call starts with a 4-byte selector derived from the function signature:

import { Function } from 'tevm';
import { Keccak256 } from 'tevm';

// Function signature: name + parameter types (no spaces, no names)
const signature = "transfer(address,uint256)";

// Hash signature with keccak256
const hash = Keccak256.hash(signature); // 0xa9059cbb...

// Take first 4 bytes for selector
const selector = hash.slice(0, 4); // 0xa9059cbb

// Or use Function.getSelector()
const transferFn = {
  type: "function",
  name: "transfer",
  inputs: [
    { type: "address", name: "to" },
    { type: "uint256", name: "amount" }
  ],
  outputs: [{ type: "bool" }]
};

const sel = Function.getSelector(transferFn); // 0xa9059cbb

Why Selectors Matter

The EVM uses selectors to route function calls. Calldata format:

[4 bytes selector][encoded parameters]

Example calling transfer(0x742d..., 1000):

0xa9059cbb  // Selector for transfer(address,uint256)
0000000000000000000000742d35Cc6634C0532925a3b844Bc9e7595f51e3e  // address (32 bytes)
00000000000000000000000000000000000000000000000000000000000003e8  // uint256(1000)

Encoding Rules

ABI encoding packs data into 32-byte words with specific alignment rules.

Fixed-Size Types

Types with known size encode directly into 32-byte slots:

uint256
address
bool
bytes32

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setValue",
  inputs: [{ type: "uint256", name: "value" }]
};

const encoded = Function.encodeParams(fn, [42n]);
// 0x000000000000000000000000000000000000000000000000000000000000002a
//   ^-- 32 bytes, right-aligned

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setOwner",
  inputs: [{ type: "address", name: "owner" }]
};

const encoded = Function.encodeParams(fn, [
  "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e"
]);
// 0x000000000000000000000000742d35Cc6634C0532925a3b844Bc9e7595f51e3e
//   ^-- Left-padded with zeros to 32 bytes

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setActive",
  inputs: [{ type: "bool", name: "active" }]
};

const encoded = Function.encodeParams(fn, [true]);
// 0x0000000000000000000000000000000000000000000000000000000000000001
//   ^-- 1 for true, 0 for false, padded to 32 bytes

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setHash",
  inputs: [{ type: "bytes32", name: "hash" }]
};

const hash = Bytes32().fill(0xaa);
const encoded = Function.encodeParams(fn, [hash]);
// 0xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
//   ^-- Exactly 32 bytes, right-aligned

Dynamic Types

Dynamic types (string, bytes, arrays) use offset-based encoding:

string
bytes
uint256[]

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setName",
  inputs: [{ type: "string", name: "name" }]
};

const encoded = Function.encodeParams(fn, ["hello"]);
// 0x0000000000000000000000000000000000000000000000000000000000000020  // Offset to data (32 bytes)
//   0000000000000000000000000000000000000000000000000000000000000005  // Length (5 bytes)
//   68656c6c6f000000000000000000000000000000000000000000000000000000  // "hello" + padding

Structure:

Offset (32 bytes) - Where data starts relative to parameter start
Length (32 bytes) - Number of bytes in data
Data (padded to 32-byte multiple) - Actual content

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setData",
  inputs: [{ type: "bytes", name: "data" }]
};

const encoded = Function.encodeParams(fn, [new Uint8Array([0xaa, 0xbb])]);
// 0x0000000000000000000000000000000000000000000000000000000000000020  // Offset
//   0000000000000000000000000000000000000000000000000000000000000002  // Length (2)
//   aabb000000000000000000000000000000000000000000000000000000000000  // Data + padding

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setNumbers",
  inputs: [{ type: "uint256[]", name: "numbers" }]
};

const encoded = Function.encodeParams(fn, [[1n, 2n, 3n]]);
// 0x0000000000000000000000000000000000000000000000000000000000000020  // Offset
//   0000000000000000000000000000000000000000000000000000000000000003  // Length (3 elements)
//   0000000000000000000000000000000000000000000000000000000000000001  // Element 0
//   0000000000000000000000000000000000000000000000000000000000000002  // Element 1
//   0000000000000000000000000000000000000000000000000000000000000003  // Element 2

Complete Function Call Example

Encode complete calldata for transfer(address,uint256):

import { Function } from 'tevm';

const transferFn = {
  type: "function",
  name: "transfer",
  stateMutability: "nonpayable",
  inputs: [
    { type: "address", name: "to" },
    { type: "uint256", name: "amount" }
  ],
  outputs: [{ type: "bool" }]
};

// Encode function call
const calldata = Function.encodeParams(transferFn, [
  "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
  1000n
]);

console.log(calldata);
// Result (68 bytes total):
// 0xa9059cbb  // Function selector (4 bytes)
//   000000000000000000000000742d35Cc6634C0532925a3b844Bc9e7595f51e3e  // address
//   00000000000000000000000000000000000000000000000000000000000003e8  // uint256(1000)

Use this calldata when making contract calls via RPC or transaction.

Decoding Return Values

Decode function return data after execution:

import { Function } from 'tevm';

const balanceOfFn = {
  type: "function",
  name: "balanceOf",
  inputs: [{ type: "address", name: "owner" }],
  outputs: [{ type: "uint256", name: "balance" }]
};

// Simulate return data from contract
const returnData = new Uint8Array([
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0xe8  // 1000 in hex
]);

const result = Function.decodeResult(balanceOfFn, returnData);
console.log(result); // [1000n]

Multiple Parameters

Mixed fixed and dynamic types require offset tracking:

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "register",
  inputs: [
    { type: "address", name: "user" },      // Fixed (32 bytes)
    { type: "string", name: "username" },   // Dynamic (offset)
    { type: "uint256", name: "age" }        // Fixed (32 bytes)
  ]
};

const encoded = Function.encodeParams(fn, [
  "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
  "alice",
  25n
]);

// Structure:
// [selector - 4 bytes]
// [address - 32 bytes, inline]
// [string offset - 32 bytes, points to 0x60]
// [age - 32 bytes, inline]
// [string length - 32 bytes]
// [string data - padded to 32 byte multiple]

Encoding Layout

Offset  Content
------  -------
0x00    a9059cbb...                                                        [selector]
0x04    000000000000000000000000742d35Cc6634C0532925a3b844Bc9e7595f51e3e  [address inline]
0x24    0000000000000000000000000000000000000000000000000000000000000060  [string offset → 0x64]
0x44    0000000000000000000000000000000000000000000000000000000000000019  [uint256(25) inline]
0x64    0000000000000000000000000000000000000000000000000000000000000005  [string length]
0x84    616c696365000000000000000000000000000000000000000000000000000000  [string data "alice"]

Constructor Encoding

Encode constructor parameters for contract deployment:

import { Constructor } from 'tevm';

const constructorDef = {
  type: "constructor",
  inputs: [
    { type: "string", name: "name" },
    { type: "string", name: "symbol" },
    { type: "uint8", name: "decimals" }
  ]
};

const encoded = Constructor.encodeParams(constructorDef, [
  "MyToken",
  "MTK",
  18
]);

// Append this to bytecode when deploying:
// const deployData = concat([contractBytecode, encoded]);

Event Encoding

Events split data into indexed topics (for filtering) and non-indexed data (for details).

Event Structure

import { Event } from 'tevm';

const transferEvent = {
  type: "event",
  name: "Transfer",
  inputs: [
    { type: "address", name: "from", indexed: true },   // Topic 1
    { type: "address", name: "to", indexed: true },     // Topic 2
    { type: "uint256", name: "value", indexed: false }  // Data
  ]
};

Encoding Event Topics

Topic 0 is always the event signature hash:

import { Event, Keccak256 } from 'tevm';

// Get event signature hash (topic 0)
const signature = Event.getSignature(transferEvent);
// "Transfer(address,address,uint256)"

const topic0 = Keccak256.hash(signature);
// 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef

// Encode indexed parameters as topics
const topics = Event.encodeTopics(transferEvent, {
  from: "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
  to: "0x5B38Da6a701c568545dCfcB03FcB875f56beddC4"
});

console.log(topics);
// [
//   "0xddf252ad...",  // Topic 0: event signature
//   "0x000000000000000000000000742d35cc6634c0532925a3b844bc9e7595f51e3e",  // from
//   "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4"   // to
// ]

Decoding Event Logs

Parse log data and topics back to values:

import { Event } from 'tevm';

const transferEvent = {
  type: "event",
  name: "Transfer",
  inputs: [
    { type: "address", name: "from", indexed: true },
    { type: "address", name: "to", indexed: true },
    { type: "uint256", name: "value", indexed: false }
  ]
};

// Simulate log from blockchain
const logData = new Uint8Array([
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0xe8  // value: 1000
]);

const logTopics = [
  "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
  "0x000000000000000000000000742d35cc6634c0532925a3b844bc9e7595f51e3e",
  "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4"
];

const decoded = Event.decodeLog(transferEvent, logData, logTopics);
console.log(decoded);
// {
//   from: "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
//   to: "0x5B38Da6a701c568545dCfcB03FcB875f56beddC4",
//   value: 1000n
// }

Why Indexed Parameters?

Indexed parameters become topics, enabling efficient filtering:

// Filter for transfers TO specific address
const logsToAddress = await provider.getLogs({
  address: tokenAddress,
  topics: [
    Event.getSignature(transferEvent),
    null,  // from: any
    "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4"  // to: filter
  ]
});

Limit: Maximum 3 indexed parameters per event (topics 1-3).

Tuple Encoding

Tuples (structs) encode as grouped parameters:

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setUser",
  inputs: [
    {
      type: "tuple",
      name: "user",
      components: [
        { type: "address", name: "addr" },
        { type: "string", name: "name" },
        { type: "uint256", name: "balance" }
      ]
    }
  ]
};

const encoded = Function.encodeParams(fn, [
  {
    addr: "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
    name: "alice",
    balance: 1000n
  }
]);

// Tuple contents encode like function parameters:
// [address inline]
// [string offset]
// [balance inline]
// [string data]

Array of Tuples

import { Function } from 'tevm';

const fn = {
  type: "function",
  name: "setUsers",
  inputs: [
    {
      type: "tuple[]",
      name: "users",
      components: [
        { type: "address", name: "addr" },
        { type: "uint256", name: "balance" }
      ]
    }
  ]
};

const encoded = Function.encodeParams(fn, [
  [
    { addr: "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e", balance: 1000n },
    { addr: "0x5B38Da6a701c568545dCfcB03FcB875f56beddC4", balance: 2000n }
  ]
]);

// Structure:
// [array offset]
// [array length]
// [tuple 0 data]
// [tuple 1 data]

Visual Encoding Example

Encoding register(address,string,uint256) with values:

address: 0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e
string: "alice"
uint256: 25

┌─────────────────────────────────────────────────────────────────┐
│ Calldata Structure                                              │
├─────────────────────────────────────────────────────────────────┤
│ 0x00: a9059cbb        │ Function selector                       │
├─────────────────────────────────────────────────────────────────┤
│ 0x04: 000000...51e3e │ address (fixed, 32 bytes)              │
├─────────────────────────────────────────────────────────────────┤
│ 0x24: 000000...00060 │ string offset → points to 0x64         │
├─────────────────────────────────────────────────────────────────┤
│ 0x44: 000000...00019 │ uint256(25) (fixed, 32 bytes)          │
├─────────────────────────────────────────────────────────────────┤
│ 0x64: 000000...00005 │ string length (5 bytes)                │
├─────────────────────────────────────────────────────────────────┤
│ 0x84: 616c69...00000 │ "alice" + padding                      │
└─────────────────────────────────────────────────────────────────┘

Key:
• Fixed types encode inline at their position
• Dynamic types store offset, actual data follows all fixed types
• Offsets are relative to first parameter (after selector)
• All values padded to 32-byte boundaries

Common Use Cases

Calling Contract Functions

import { Function } from 'tevm';

// Define function
const transferFn = {
  type: "function",
  name: "transfer",
  inputs: [
    { type: "address", name: "to" },
    { type: "uint256", name: "amount" }
  ],
  outputs: [{ type: "bool" }]
};

// Encode calldata
const calldata = Function.encodeParams(transferFn, [
  "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e",
  1000n
]);

// Use in transaction
const tx = {
  to: tokenAddress,
  data: calldata,  // Send this to contract
  value: 0n
};

Parsing Event Logs

import { Event } from 'tevm';

const transferEvent = {
  type: "event",
  name: "Transfer",
  inputs: [
    { type: "address", name: "from", indexed: true },
    { type: "address", name: "to", indexed: true },
    { type: "uint256", name: "value", indexed: false }
  ]
};

// Get logs from RPC
const logs = await provider.getLogs({
  address: tokenAddress,
  topics: [Event.getSignature(transferEvent)]
});

// Decode each log
const transfers = logs.map(log =>
  Event.decodeLog(transferEvent, log.data, log.topics)
);

console.log(transfers);
// [
//   { from: "0x...", to: "0x...", value: 1000n },
//   { from: "0x...", to: "0x...", value: 2000n }
// ]

Handling Contract Errors

import { AbiError } from 'tevm';

const insufficientBalanceError = {
  type: "error",
  name: "InsufficientBalance",
  inputs: [
    { type: "uint256", name: "balance" },
    { type: "uint256", name: "required" }
  ]
};

try {
  // Contract call that reverts
  await contract.call();
} catch (error) {
  // Decode revert data
  const decoded = AbiError.decodeParams(
    insufficientBalanceError,
    error.data
  );

  console.log(`Balance: ${decoded.balance}, Required: ${decoded.required}`);
}

Type Safety

Tevm provides full TypeScript type inference:

import { Function } from 'tevm';

const balanceOfFn = {
  type: "function",
  name: "balanceOf",
  inputs: [{ type: "address", name: "owner" }],
  outputs: [{ type: "uint256", name: "balance" }]
} as const;  // Use 'as const' for inference

// TypeScript knows parameters must be [address]
const encoded = Function.encodeParams(balanceOfFn, [
  "0x742d35Cc6634C0532925a3b844Bc9e7595f51e3e"
  // "invalid"  // TypeScript error - must be address
]);

// TypeScript knows result is [bigint]
const result = Function.decodeResult(balanceOfFn, returnData);
//    ^-- Type: [bigint]

Error Handling

ABI operations can fail with specific error types:

import {
  AbiEncodingError,
  AbiDecodingError,
  AbiParameterMismatchError
} from 'tevm';

try {
  const encoded = Function.encodeParams(fn, [/* params */]);
} catch (error) {
  if (error instanceof AbiParameterMismatchError) {
    // Wrong number of parameters
    console.error("Parameter count mismatch");
  } else if (error instanceof AbiEncodingError) {
    // Invalid value (out of range, wrong type)
    console.error("Encoding failed:", error.message);
  }
}

Resources

Solidity ABI Specification - Official encoding specification
Contract ABI Specification - JSON ABI format
EIP-712 - Typed structured data hashing
ABI Playground - Interactive encoder/decoder

Next Steps

Overview - Type definitions and API reference

Try it Live

​What is ABI?

​Function Selectors

​Why Selectors Matter

​Encoding Rules

​Fixed-Size Types

​Dynamic Types

​Complete Function Call Example

​Decoding Return Values

​Multiple Parameters

​Encoding Layout

​Constructor Encoding

​Event Encoding

​Event Structure

​Encoding Event Topics

​Decoding Event Logs

​Why Indexed Parameters?

​Tuple Encoding

​Array of Tuples

​Visual Encoding Example

​Common Use Cases

​Calling Contract Functions

​Parsing Event Logs

​Handling Contract Errors

​Type Safety

​Error Handling

​Resources

​Next Steps

What is ABI?

Function Selectors

Why Selectors Matter

Encoding Rules

Fixed-Size Types

Dynamic Types

Complete Function Call Example

Decoding Return Values

Multiple Parameters

Encoding Layout

Constructor Encoding

Event Encoding

Event Structure

Encoding Event Topics

Decoding Event Logs

Why Indexed Parameters?

Tuple Encoding

Array of Tuples

Visual Encoding Example

Common Use Cases

Calling Contract Functions

Parsing Event Logs

Handling Contract Errors

Type Safety

Error Handling

Resources

Next Steps