Example: ERC-20 Contract

Connecting to a Contract

A simple ERC-20 contract
// A Human-Readable ABI; any supported ABI format could be used const abi = [ // Read-Only Functions "function balanceOf(address owner) view returns (uint256)", "function decimals() view returns (uint8)", "function symbol() view returns (string)", // Authenticated Functions "function transfer(address to, uint amount) returns (boolean)", // Events "event Transfer(address indexed from, address indexed to, uint amount)" ]; // This can be an address or an ENS name const address = "dai.tokens.ethers.eth"; // An example Provider const provider = ethers.getDefaultProvider(); // An example Signer const signer = ethers.Wallet.createRandom().connect(provider); // Read-Only; By connecting to a Provider, allows: // - Any constant function // - Querying Filters // - Populating Unsigned Transactions for non-constant methods // - Estimating Gas for non-constant (as an anonymous sender) // - Static Calling non-constant methods (as anonymous sender) const erc20 = new ethers.Contract(address, abi, provider); // Read-Write; By connecting to a Signer, allows: // - Everything from Read-Only (except as Signer, not anonymous) // - Sending transactions for non-constant functions const erc20_rw = new ethers.Contract(address, abi, signer)

ERC20Contract inherits Contract

new ethers.Contract( address , abi , providerOrSigner )

See the above code example for creating an Instance which will (in addition to the Contact methods and properties) automatically add the additional properties defined in abi to a Contract connected to address using the providerOrSigner.

Properties(inheritted from Contract)

erc20.address string< Address >

This is the address (or ENS name) the contract was constructed with.

erc20.resolvedAddress string< Address >

This is a promise that will resolve to the address the Contract object is attached to. If an Address was provided to the constructor, it will be equal to this; if an ENS name was provided, this will be the resolved address.

erc20.deployTransaction TransactionResponse

If the Contract object is the result of a ContractFactory deployment, this is the transaction which was used to deploy the contract.

erc20.interface Interface

This is the ABI as an Interface.

erc20.provider Provider

If a provider was provided to the constructor, this is that provider. If a signer was provided that had a Provider, this is that provider.

erc20.signer Signer

If a signer was provided to the constructor, this is that signer.

Methods(inheritted from Contract)

erc20.attach( addressOrName ) Contract

Returns a new instance of the Contract attached to a new address. This is useful if there are multiple similar or identical copies of a Contract on the network and you wish to interact with each of them.

erc20.connect( providerOrSigner ) Contract

Returns a new instance of the Contract, but connected to providerOrSigner.

By passing in a Provider, this will return a downgraded Contract which only has read-only access (i.e. constant calls).

By passing in a Signer. this will return a Contract which will act on behalf of that signer.

erc20.deployed( ) Promise< Contract >
Contract.isIndexed( value ) boolean

Events(inheritted from Contract)

erc20.queryFilter( event [ , fromBlockOrBlockHash [ , toBlock ] ) Promise< Array< Event > >

Return Events that match the event.

erc20.listenerCount( [ event ] ) number

Return the number of listeners that are subscribed to event. If no event is provided, returns the total count of all events.

erc20.listeners( event ) Array< Listener >

Return a list of listeners that are subscribed to event.

erc20.off( event , listener ) this

Unsubscribe listener to event.

erc20.on( event , listener ) this

Subscribe to event calling listener when the event occurs.

erc20.once( event , listener ) this

Subscribe once to event calling listener when the event occurs.

erc20.removeAllListeners( [ event ] ) this

Unsubscribe all listeners for event. If no event is provided, all events are unsubscribed.

Meta-Class Methods(added at Runtime)

Since the Contract is a Meta-Class, the methods available here depend on the ABI which was passed into the Contract.

erc20.decimals( [ overrides ] ) Promise< number >

Returns the number of decimal places used by this ERC-20 token. This can be used with parseUnits when taking input from the user or [formatUnits](utils-formatunits] when displaying the token amounts in the UI.

erc20.balanceOf( owner [ , overrides ] ) Promise< BigNumber >

Returns the balance of owner for this ERC-20 token.

erc20.symbol( [ overrides ] ) Promise< string >

Returns the symbol of the token.

erc20_rw.transfer( target , amount [ , overrides ] ) Promise< TransactionResponse >

Transfers amount tokens to target from the current signer. The return value (a boolean) is inaccessible during a write operation using a transaction. Other techniques (such as events) are required if this value is required. On-chain contracts calling the transfer function have access to this result, which is why it is possible.

erc20.callStatic.transfer( target , amount [ , overrides ] ) Promise< boolean >

Performs a dry-run of transferring amount tokens to target from the current signer, without actually signing or sending a transaction.

This can be used to preflight check that a transaction will be successful.

erc20.estimateGas.transfer( target , amount [ , overrides ] ) Promise< BigNumber >

Returns an estimate for how many units of gas would be required to transfer amount tokens to target.

erc20.populateTransaction.transfer( target , amount [ , overrides ] ) Promise< UnsignedTx >

Returns an UnsignedTransaction which could be signed and submitted to the network to transaction amount tokens to target.

Note on Estimating and Static Calling

When you perform a static call, the current state is taken into account as best as Ethereum can determine. There are many cases where this can provide false positives and false negatives. The eventually consistent model of the blockchain also means there are certain consistency modes that cannot be known until an actual transaction is attempted.

Meta-Class Filters(added at Runtime)

Since the Contract is a Meta-Class, the methods available here depend on the ABI which was passed into the Contract.

erc20.filters.Transfer( [ fromAddress [ , toAddress ] ] ) Filter

Returns a new Filter which can be used to query or to subscribe/unsubscribe to events.

If fromAddress is null or not provided, then any from address matches. If toAddress is null or not provided, then any to address matches.