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Multicall.js <img width="100" align="right" alt="Multicall" src="https://user-images.githubusercontent.com/304108/55666937-320cb180-5888-11e9-907b-48ba66150523.png" />

Multicall.js is a lightweight JavaScript library for interacting with the multicall smart contract.

Multicall allows multiple smart contract constant function calls to be grouped into a single call and the results aggregated into a single result. This reduces the number of separate JSON RPC requests that need to be sent over the network if using a remote node like Infura, and provides the guarantee that all values returned are from the same block. The latest block number is also returned along with the aggregated results.

Summary

• Get the return value(s) of multiple smart contract function calls in a single call
• Guarantee that all values are from the same block
• Use watchers to poll for multiple blockchain state variables/functions
• Get updates when a watcher detects state has changed
• Results from out of sync nodes are automatically ignored
• Get new block updates

Installation

yarn add @makerdao/multicall

Usage

import { createWatcher } from '@makerdao/multicall';

// Contract addresses used in this example
const MKR_TOKEN = '0xaaf64bfcc32d0f15873a02163e7e500671a4ffcd';
const MKR_WHALE = '0xdb33dfd3d61308c33c63209845dad3e6bfb2c674';
const MKR_FISH = '0x2dfcedcb401557354d0cf174876ab17bfd6f4efd';

// Preset can be 'mainnet', 'kovan', 'rinkeby', 'goerli' or 'xdai'
const config = { preset: 'kovan' };

// Create watcher
const watcher = createWatcher(
  [
    {
      target: MKR_TOKEN,
      call: ['balanceOf(address)(uint256)', MKR_WHALE],
      returns: [['BALANCE_OF_MKR_WHALE', val => val / 10 ** 18]]
    }
  ],
  config
);

// Subscribe to state updates
watcher.subscribe(update => {
console.log(`Update: ${update.type} = ${update.value}`);
});

// Subscribe to batched state updates
watcher.batch().subscribe(updates => {
  // Handle batched updates here
  // Updates are returned as { type, value } objects, e.g:
  // { type: 'BALANCE_OF_MKR_WHALE', value: 70000 }
});

// Subscribe to new block number updates
watcher.onNewBlock(blockNumber => {
  console.log('New block:', blockNumber);
});

// Start the watcher polling
watcher.start();

// The JSON RPC URL and multicall contract address can also be specified in the config:
const config = {
  rpcUrl: 'https://kovan.infura.io',
  multicallAddress: '0xc49ab4d7de648a97592ed3d18720e00356b4a806'
};

// Update the watcher calls using tap()
const fetchWaiter = watcher.tap(calls => [
  // Pass back existing calls...
  ...calls,
  // ...plus new calls
  {
    target: MKR_TOKEN,
    call: ['balanceOf(address)(uint256)', MKR_FISH],
    returns: [['BALANCE_OF_MKR_FISH', val => val / 10 ** 18]]
  }
]);
// This promise resolves when the first fetch completes
fetchWaiter.then(() => {
  console.log('Initial fetch completed');
});

// Recreate the watcher with new calls and config (allowing the network to be changed)
const config = { preset: 'mainnet' };
watcher.recreate(
  [
    {
      target: MKR_TOKEN,
      call: ['balanceOf(address)(uint256)', MKR_WHALE],
      returns: [['BALANCE_OF_MKR_WHALE', val => val / 10 ** 18]]
    }
  ],
  config
);

Helper Functions

Special variables and functions (e.g. addr.balance, block.blockhash, block.timestamp) can be accessed by calling their corresponding helper function. To call these helper functions simply omit the target property (and it will default to multicall's contract address).

const watcher = createWatcher(
  [
    {
      call: [
        'getEthBalance(address)(uint256)', 
        '0x72776bb917751225d24c07d0663b3780b2ada67c'
      ],
      returns: [['ETH_BALANCE', val => val / 10 ** 18]]
    },
    {
      call: ['getBlockHash(uint256)(bytes32)', 11482494],
      returns: [['SPECIFIC_BLOCK_HASH_0xFF4DB']]
    },
    {
      call: ['getLastBlockHash()(bytes32)'],
      returns: [['LAST_BLOCK_HASH']]
    },
    {
      call: ['getCurrentBlockTimestamp()(uint256)'],
      returns: [['CURRENT_BLOCK_TIMESTAMP']]
    },
    {
      call: ['getCurrentBlockDifficulty()(uint256)'],
      returns: [['CURRENT_BLOCK_DIFFICULTY']]
    },
    {
      call: ['getCurrentBlockGasLimit()(uint256)'],
      returns: [['CURRENT_BLOCK_GASLIMIT']]
    },
    {
      call: ['getCurrentBlockCoinbase()(address)'],
      returns: [['CURRENT_BLOCK_COINBASE']]
    }
  ],
  { preset: 'kovan' }
);

Examples

Check out this CodePen example for a working front-end example.

To run the example in the project, first clone this repo:

git clone https://github.com/makerdao/multicall.js

Then install the dependencies:

yarn

Finally run the example script (examples/es-example.js):

yarn example

Test

To run tests use:

yarn test