Contracts

A Contract is an abstraction of an executable program on the Ethereum Blockchain. A Contract has code (called byte code) as well as allocated long-term memory (called storage). Every deployed Contract has an address, which is used to connect to it so that it may be sent messages to call its methods.

A Contract can emit Events, which can be efficiently observed by applications to be notified when a contract has performed specific operation. Events cannot be read by a Contract.

There are two types of methods that can be called on a Contact:

A Constant method may not add, remove or change any data in the storage, nor log any events, and may only call Constant methods on other contracts. These methods are free (no Ether is required) to call. The result from them may also be returned to the caller.

A Non-Constant method requires a fee (in Ether) to be paid, but may perform any state-changing operation desired, log events, send ether and call Non-Constant methods on other Contracts. These methods cannot return their result to the caller. These methods must be triggered by a transaction, sent by an Externally Owned Account (EOA) either directly or indirectly (i.e. called from another contract), and are required to be mined before the effects are present. Therefore, the duration required for these operations can vary widely, and depend on the transaction gas price, network congestion and miner priority heuristics.

The Contract API provides simple way to connect to a Contract and call its methods, as functions on a JavaScript object, handling all the binary protocol conversion, internal name mangling and topic construction. This allows a Contract object to be used like any standard JavaScript object, without having to worry about the low-level details of the Ethereum Virtual Machine or Blockchain.

The Contract object is a meta-class, which is a class that defines a Class at run-time. The Contract definition (called an Application Binary Interface, or ABI) can be provided and the available methods and events will be dynamically added to the object.

Throughout this document, we will refer to the following Contract.

SimpleStorage Contract
pragma solidity ^0.4.24;

contract SimpleStorage {

    event ValueChanged(address indexed author, string oldValue, string newValue);

    string _value;

    constructor(string value) public {
        emit ValueChanged(msg.sender, _value, value);
        _value = value;
    }

    function getValue() view public returns (string) {
        return _value;
    }

    function setValue(string value) public {
        emit ValueChanged(msg.sender, _value, value);
        _value = value;
    }
}

Deploying a Contract

To deploy a contract to the Ethereum network, a ContractFactory can be created which manages the Contract bytecode and Application Binary Interface (ABI), usually generated from the Solidity compiler.

Creating a Contract Factory

new ethers . ContractFactory ( abi , bytecode [ , signer ] )
Creates a factory for deployment of the Contract with bytecode, and the constructor defined in the abi. The signer will be used to send any deployment transaction.
ethers . ContractFactory . fromSolidity ( compilerOutput [ , signer ] )
Creates a ContractFactory from the compilerOutput of the Solidity compiler or from the Truffle JSON. (i.e. output.contracts['SimpleStorage.sol'].SimpleStorage)
prototype . connect ( signer )   =>   ContractFactory
Create a new instance of the ContractFactory, connected to the new signer.

Prototype

prototype . bytecode
The Contract executable byte code..
prototype . interface
The Contract Application Binary Interface (ABI).
prototype . signer
The Signer that will be used to send transactions to the network. If this is null, deploy() cannot be called.

Connecting

prototype . attach ( address )   =>   Contract
Connect to an existing instance of this Contract at address using the Contract Interface and Signer.

Deployment

prototype . deploy ( … )   =>   Promise<Contract>

Creates a transaction to deploy the transaction and sends it to the network using the contract Signer, returning a Promise that resolves to a Contract. The transaction is available as contract.deployTransaction.

Keep in mind that the Contract may not be mined immediately. The contract.deployed() function will return a Promise which will resolve once the contract is deployed, or reject if there was an error during deployment.

prototype . getDeployTransaction ( … )   =>   UnsignedTransaction
Returns the transaction required to deploy the Contract with the provided constructor arguments. This is often useful for signing offline transactions or analysis tools.
Deploy a Contract
const ethers = require('ethers');

// The Contract interface
let abi = [
    "event ValueChanged(address indexed author, string oldValue, string newValue)",
    "constructor(string value)",
    "function getValue() view returns (string value)",
    "function setValue(string value)"
];

// The bytecode from Solidity, compiling the above source
let bytecode = "0x608060405234801561001057600080fd5b506040516105bd3803806105bd8339" +
                 "8101604081815282518183526000805460026000196101006001841615020190" +
                 "91160492840183905293019233927fe826f71647b8486f2bae59832124c70792" +
                 "fba044036720a54ec8dacdd5df4fcb9285919081906020820190606083019086" +
                 "9080156100cd5780601f106100a2576101008083540402835291602001916100" +
                 "cd565b820191906000526020600020905b815481529060010190602001808311" +
                 "6100b057829003601f168201915b505083810382528451815284516020918201" +
                 "9186019080838360005b838110156101015781810151838201526020016100e9" +
                 "565b50505050905090810190601f16801561012e578082038051600183602003" +
                 "6101000a031916815260200191505b5094505050505060405180910390a28051" +
                 "610150906000906020840190610157565b50506101f2565b8280546001816001" +
                 "16156101000203166002900490600052602060002090601f0160209004810192" +
                 "82601f1061019857805160ff19168380011785556101c5565b82800160010185" +
                 "5582156101c5579182015b828111156101c55782518255916020019190600101" +
                 "906101aa565b506101d19291506101d5565b5090565b6101ef91905b80821115" +
                 "6101d157600081556001016101db565b90565b6103bc806102016000396000f3" +
                 "0060806040526004361061004b5763ffffffff7c010000000000000000000000" +
                 "0000000000000000000000000000000000600035041663209652558114610050" +
                 "57806393a09352146100da575b600080fd5b34801561005c57600080fd5b5061" +
                 "0065610135565b60408051602080825283518183015283519192839290830191" +
                 "85019080838360005b8381101561009f57818101518382015260200161008756" +
                 "5b50505050905090810190601f1680156100cc57808203805160018360200361" +
                 "01000a031916815260200191505b509250505060405180910390f35b34801561" +
                 "00e657600080fd5b506040805160206004803580820135601f81018490048402" +
                 "8501840190955284845261013394369492936024939284019190819084018382" +
                 "80828437509497506101cc9650505050505050565b005b600080546040805160" +
                 "20601f6002600019610100600188161502019095169490940493840181900481" +
                 "0282018101909252828152606093909290918301828280156101c15780601f10" +
                 "610196576101008083540402835291602001916101c1565b8201919060005260" +
                 "20600020905b8154815290600101906020018083116101a457829003601f1682" +
                 "01915b505050505090505b90565b604080518181526000805460026000196101" +
                 "00600184161502019091160492820183905233927fe826f71647b8486f2bae59" +
                 "832124c70792fba044036720a54ec8dacdd5df4fcb9285918190602082019060" +
                 "60830190869080156102715780601f1061024657610100808354040283529160" +
                 "200191610271565b820191906000526020600020905b81548152906001019060" +
                 "200180831161025457829003601f168201915b50508381038252845181528451" +
                 "60209182019186019080838360005b838110156102a557818101518382015260" +
                 "200161028d565b50505050905090810190601f1680156102d257808203805160" +
                 "01836020036101000a031916815260200191505b509450505050506040518091" +
                 "0390a280516102f49060009060208401906102f8565b5050565b828054600181" +
                 "600116156101000203166002900490600052602060002090601f016020900481" +
                 "019282601f1061033957805160ff1916838001178555610366565b8280016001" +
                 "0185558215610366579182015b82811115610366578251825591602001919060" +
                 "01019061034b565b50610372929150610376565b5090565b6101c991905b8082" +
                 "1115610372576000815560010161037c5600a165627a7a723058202225a35c50" +
                 "7b31ac6df494f4be31057c7202b5084c592bdb9b29f232407abeac0029";


// Connect to the network
let provider = ethers.getDefaultProvider('ropsten');

// Load the wallet to deploy the contract with
let privateKey = '0x0123456789012345678901234567890123456789012345678901234567890123';
let wallet = new ethers.Wallet(privateKey, provider);

// Create an instance of a Contract Factory
let factory = new ethers.ContractFactory(abi, bytecode, wallet);

// Notice we pass in "Hello World" as the parameter to the constructor
let contract = await factory.deploy("Hello World");

// The address the Contract WILL have once mined
// See: https://ropsten.etherscan.io/address/0x2bd9aaa2953f988153c8629926d22a6a5f69b14e
console.log(contract.address);
// "0x2bD9aAa2953F988153c8629926D22A6a5F69b14E"

// The transaction that was sent to the network to deploy the Contract
// See: https://ropsten.etherscan.io/tx/0x159b76843662a15bd67e482dcfbee55e8e44efad26c5a614245e12a00d4b1a51
console.log(contract.deployTransaction.hash);
// "0x159b76843662a15bd67e482dcfbee55e8e44efad26c5a614245e12a00d4b1a51"

// The contract is NOT deployed yet; we must wait until it is mined
await contract.deployed()

// Done! The contract is deployed.

Connecting to Existing Contracts

Once a Contract has been deployed, it can be connected to using the Contract object.

Connecting to a Contract

new ethers . Contract ( addressOrName , abi , providerOrSigner )

Connects to the contract at addressOrName defined by abi, connected as providerOrSigner.

For supported formats for abi, see Contract ABI.

For access capabilities and restrictions, see Providers vs Signers

Connecting to an existing Contract
const ethers = require('ethers');

// The Contract interface
let abi = [
    "event ValueChanged(address indexed author, string oldValue, string newValue)",
    "constructor(string value)",
    "function getValue() view returns (string value)",
    "function setValue(string value)"
];

// Connect to the network
let provider = ethers.getDefaultProvider();

// The address from the above deployment example
let contractAddress = "0x2bD9aAa2953F988153c8629926D22A6a5F69b14E";

// We connect to the Contract using a Provider, so we will only
// have read-only access to the Contract
let contract = new ethers.Contract(contractAddress, abi, provider);
Calling a read-only Constant Method
// Get the current value
let currentValue = await contract.getValue();

console.log(currentValue);
// "Hello World"
Calling a Non-Constant Method
// A Signer from a private key
let privateKey = '0x0123456789012345678901234567890123456789012345678901234567890123';
let wallet = new ethers.Wallet(privateKey, provider);

// Create a new instance of the Contract with a Signer, which allows
// update methods
let contractWithSigner = contract.connect(wallet);
// ... OR ...
// let contractWithSigner = new Contract(contractAddress, abi, wallet)

// Set a new Value, which returns the transaction
let tx = await contractWithSigner.setValue("I like turtles.");

// See: https://ropsten.etherscan.io/tx/0xaf0068dcf728afa5accd02172867627da4e6f946dfb8174a7be31f01b11d5364
console.log(tx.hash);
// "0xaf0068dcf728afa5accd02172867627da4e6f946dfb8174a7be31f01b11d5364"

// The operation is NOT complete yet; we must wait until it is mined
await tx.wait();

// Call the Contract's getValue() method again
let newValue = await contract.getValue();

console.log(currentValue);
// "I like turtles."
Listening to Events
contract.on("ValueChanged", (author, oldValue, newValue, event) => {
    // Called when anyone changes the value

    console.log(author);
    // "0x14791697260E4c9A71f18484C9f997B308e59325"

    console.log(oldValue);
    // "Hello World"

    console.log(newValue);
    // "Ilike turtles."

    // See Event Emitter below for all properties on Event
    console.log(event.blockNumber);
    // 4115004
});
Filtering an Events
// A filter that matches my Signer as the author
let filter = contract.filters.ValueChanged(wallet.address);

contract.on(filter, (author, oldValue, newValue, event) => {
    // Called ONLY when your account changes the value
});

Prototype

prototype . address
The address (or ENS name) of the contract.
prototype . deployTransaction
If the contract was deployed by a ContractFactory, this is the transaction used to deploy it, otherwise it is null.
prototype . interface
The Interface meta-class of the parsed ABI. Generally, this should not need to be accessed directly.

Additional properties will be added to the prototype at run-time, based on the ABI provided, see Contract Meta-Class.


Waiting for Deployment

prototype . deployed ( )   =>   Promise<Contract>
If the contract is the result of deploy(), returns a Promise that resolves to the contract once it has been mined, or rejects if the contract failed to deploy. If the contract has been deployed already, this will return a Promise that resolves once the on-chain code has been confirmed.

Meta-Class Properties

Since a Contract is dynamic and loaded at run-time, many of the properties that will exist on a Contract are determined at run-time from the Contract ABI.

Contract Methods

All functions populated from the ABI are also included on the contract object directly, for example contract.functions.getValue() can also be called using contract.getValue().

prototype . functions . functionName

An object that maps each ABI function name to a function that will either call (for constant functions) or sign and send a transaction (for non-constant functions)

Calling a Constant function requires either a Provider or a Signer with a Provider.

Calling a Non-Constant function (i.e. sending a transaction) requires a Signer.

prototype . estimate . functionName
An object that maps each ABI function name to a function that will estimate the cost the provided parameters.

Contract Event Filters

Filters allow for a flexible and efficient way to fetch only a subset of the events that match specific criteria. The filters property contains a function for every Event in the ABI that computes a Filter for a given set of values. The null matches any value.

prototype . filters . eventname
A function that generates filters that can be listened to, using the on(eventName, ...) function, filtered by the Event values.
Filtering Events
// A filter from me to anyone
let filterFromMe = contract.filters.Transfer(myAddress);

// A filter from anyone to me
let filterToMe = contract.filters.Transfer(null, myAddress);

// A filter from me AND to me
let filterFromMeToMe = contract.filters.Transfer(myAddress, myAddress);

contract.on(filterFromMe, (fromAddress, toAddress, value, event) => {
    console.log('I sent', value);
});

contract.on(filterToMe, (fromAddress, toAddress, value, event) => {
    console.log('I received', value);
});

contract.on(filterFromMeToMe, (fromAddress, toAddress, value, event) => {
    console.log('Myself to me', value);
});

Overrides

Every Contract method may take one additional (optional) parameter which specifies the transaction (or call) overrides.

Contract Transaction Overrides
// All overrides are optional
let overrides = {

    // The maximum units of gas for the transaction to use
    gasLimit: 23000,

    // The price (in wei) per unit of gas
    gasPrice: utils.parseUnits('9.0', 'gwei'),

    // The nonce to use in the transaction
    nonce: 123,

    // The amount to send with the transaction (i.e. msg.value)
    value: utils.parseEther('1.0'),

    // The chain ID (or network ID) to use
    chainId: 1

};

// Solidity: function someFunction(address addr) public
let tx = contract.someFunction(addr, overrides)
Contract Call Overrides
let overrides = {

    // The address to execute the call as
    from: "0x0123456789012345678901234567890123456789",

    // The maximum units of gas for the transaction to use
    gasLimit: 23000,

};

// Solidity: function someFunction(address addr) public pure returns (bytes32 result)
let result = contract.someFunction(addr, overrides)

Event Emitter

Each Contract supports many of the operations available from the Event Emitter API.

To listen for Events, the contract requires either a Provider or a Signer with a Provider.

Event Names

The available eventNames are:

  • string – The name of an event (e.g. “TestEvent” or “TestEvent(string, uint)”)
  • filter – See Contract Filters
  • * – All events

Event Object

All event callbacks receive the parameters specified in the ABI as well as one additional Event Object with

  • blockNumber, blockHash, transactionHash – The Block and Transaction of the Log
  • address – The contract address for the Log
  • data – The Log data
  • topics – An array of the Log topics
  • args – An array of the parsed arguments for the event
  • event – the name of the event (e.g. “Transfer”)
  • eventSignature – the full signature of the event (e.g. “Transfer(address,address,uint256)”)
  • getBlock() – A function that resolves to the Block containing the Log
  • getTransaction() – A function that resolves to the Transaction containing the Log
  • getTransactionReceipt() – A function that resolves to the Transaction Receipt containing the Log
  • removeListener() – A function that removes this callack as a listener
  • decode(data, topics) – A function that decodes data and topics into parsed arguments

Configuring Events

prototype . on ( eventName , callback )   =>   Contract
Registers callback to be called on every eventName. Returns the contract, so calls may be chained.
prototype . addEventListner ( eventName , callback )   =>   Contract
An alias for on.
prototype . once ( eventName , callback )   =>   Contract
Register callback to be called at most once, for eventName. Returns the contract, so calls may be chained.
prototype . emit ( eventName , … )   =>   boolean
Trigger all callbacks for eventName, returning true if there was at least one listener. This should generally not be called directly.
prototype . listenerCount ( [ eventName ] )   =>   number
Returns the number of callbacks registered for eventName.
prototype . listeners ( eventName )   =>   Listeners[]
Returns a list of callbacks for eventName.
prototype . removeAllListeners ( eventName )   =>   Contract
De-registers all listeners for eventName. Returns the contract, so calls may be chained.
prototype . removeListener ( eventName , callback )   =>   Contract
De-registers the specific callback for eventName. Returns the contract, so calls may be chained.
Events
contract.on("ValueChanged", (oldValue, newValue, event) => {
    console.log(oldValue, newValue);
});

Providers vs Signers

A Contract object has a notion of an “frame of reference”, which will determine what type of access and whom the Contract is enacted upon as. This is specified by the providerOrSigner parameter when connecting to a Contract.

There are three possible cases for connecting a Contract using the providerOrSigner.

providerOrSigner Operation Privileges
Provider Read-Only Access
Signer (without a provider) Write-Only Access (as account owner)
Signer (with a provider) Read and Write Access (as account owner)

The providerOrSigner is immutable, so to change the “frame of reference” to another account or provider, use the connect function.

prototype . connect ( providerOrSigner )
Create a new instance of the Contract object connected as providerOrSigner.

Types

There are many variable types available in Solidity, some which convert to and from JavaScript gracefully, and others that do not. Here are some note regarding passing and returning values in Contracts.

Bytes

Bytes are available in fixed-length or dynamic-length variants. In both cases, the values are returned as a hex string and may be passed in as either a hex string or as an arrayish.

To convert the string into an array, use the arrayify() utility function.

Integers

Integers in solidity are a fixed number of bits (aligned to the nearest byte) and are available in signed and unsigned variants.

For example, a uint256 is 256 bits (32 bytes) and unsigned. An int8 is 8 bits (1 byte) and signed.

When the type is 48 bits (6 bytes) or less, values are returned as a JavaScript Number, since Javascript Numbers are safe to use up to 53 bits.

Any types with 56 bits (7 bytes) or more will be returned as a BigNumber, even if the value is within the 53 bit safe range.

When passing numeric values in, JavaScript Numbers, hex strings or any BigNumber is acceptable (however, take care when using JavaScript Numbers and performing mathematical operations on them).

The uint and int types are aliases for uint256 and int256, respectively.

Strings

For short strings, many Contracts use a bytes32 to encode a null-terminated string representation, rather than a length-prefixed representation, so the formatBytes32String and parseBytes32String utility functions can be used to handle this conversion.

To convert between the two dynamic types, strings and bytes, the toUtf8Bytes() and toUtf8String() utility functions can be used.

Structs

Structs can be specified as Objects with their named properties, or as an Array, the same length as the struct.

Constant methods which return a single item, return that item directly. If the method returns multiple values then an object is returned which can be accessed by either the named properties or by their indices, in which case both point to the same instance.

Example Return Types
/**
 *  Contract Methods
 *
 *  function oneItem() public view returns (uint256 param1);
 *  function twoItems() public view returns (uint256 param1, uint256 param2);
 *
 */

 let resultOne = await oneItem();
 console.log(resultOne);
 // 1337

 let resultTwo = await twoItems();
 console.log(resultTwo);
 // {
 //    "param1": 1337,
 //    "param2": 42,
 //    0: 1337,
 //    1: 42,
 //    length: 2
 // }

 assert.ok(resultTwo[0] === resultTwo.param1);
 assert.ok(resultTwo[1] === resultTwo.param2);

Filtering Events

On every contract, there is a filters property, which can be used to generate an event filter. And event filter can be passed into the on(eventName) of a contract.

Find all ERC-20 transfers to myAddress
// The null field indicates any value matches, this specifies
// "any Transfer from any to myAddress"
let filter = contract.Transfer(null, myAddress);

// Listen for our filtered results
contract.on(filter, (from, to, value) => {
    console.log('I received ' + value.toString() + ' tokens from ' + from);
});

Application Binary Interface (ABI)

Each Contract has a description of its interface, which describes each function and event.

The Solidity compiler generates the ABI in a JSON format, which can be used as a JSON string or parsed as a JavaScript Object. This is generated by the compiler and can be loaded as a file, or copied into the source code.

The ABI may also be specified using Human-Readable ABI, which is much easier to use when typing in an ABI by hand, for example, as well as easier to read. This is simply an array of strings, each of which is the Solidity function or event signature.

Human-Readable ABI
let ABI = [
    "event Transfer(address from, address to, uint amount)",
    "function transfer(address to, uint amount)",
    "function symbol() returns (string)"
]