Example: ERC-20 Contract

The concept of Meta-Classes is somewhat confusing, so we will go over a short example.

A meta-class is a class which is defined at run-time. A Contract is specified by an Application Binary Interface (ABI), which describes the methods and events it has. This description is passed the the Contract object at run-time, and it creates a new Class, adding all the methods defined in the ABI at run-time.

Deploying a Contract

Most often, any contract you will need to interact with will already be deployed to the blockchain, but for this example will will first deploy the contract.

new ethers.ContractFactory( abi , bytecode , signer )

Create a new ContractFactory which can deploy a contract to the blockchain.

const bytecode = "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"; // A Human-Readable ABI; we only need to specify relevant fragments, // in the case of deployment this means the constructor const abi = [ "constructor(uint totalSupply)" ]; const factory = new ethers.ContractFactory(abi, bytecode, signer) // Deploy, setting total supply to 100 tokens (assigned to the deployer) const contract = await factory.deploy(parseUnits("100")); // The contract is not currentl live on the network yet, however // its address is ready for us contract.address // '0x4f3b15e4421902c09895fB12c8e0B8821134eA39' // Wait until the contract has been deployed before interacting // with it; returns the receipt for the deployemnt transaction await contract.deployTransaction.wait(); // { // blockHash: '0x62fd00943531646ad700ee14a3e63c2ff553497439db486ebf04a1c5c624ae9b', // blockNumber: 45, // byzantium: true, // confirmations: 1, // contractAddress: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // cumulativeGasUsed: { BigNumber: "250842" }, // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // gasUsed: { BigNumber: "250842" }, // logs: [ // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // blockHash: '0x62fd00943531646ad700ee14a3e63c2ff553497439db486ebf04a1c5c624ae9b', // blockNumber: 45, // data: '0x0000000000000000000000000000000000000000000000056bc75e2d63100000', // logIndex: 0, // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000000000000000000000000000000000000000000000', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78' // ], // transactionHash: '0xe05aa74209e5b928013952153dd231b42a8fdffb4bcc103e47c59e80d78a8629', // transactionIndex: 0 // } // ], // logsBloom: '0x00000001000000000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000008000000000000000008000000000000000000000000000000000000000000000000020000000000000000000800000000000000000000000010000000000000000020000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000020000000000000000000000000000000000001000000000000000000000000000000', // status: 1, // to: null, // transactionHash: '0xe05aa74209e5b928013952153dd231b42a8fdffb4bcc103e47c59e80d78a8629', // transactionIndex: 0, // type: 0 // }

Connecting to a Contract

ERC20Contract inherits Contract

new ethers.Contract( address , abi , providerOrSigner )

Creating a new instance of a Contract connects to an existing contract by specifying its address on the blockchain, its abi (used to populate the class' methods) a providerOrSigner.

If a Provider is given, the contract has only read-only access, while a Signer offers access to state manipulating methods.

// A Human-Readable ABI; for interacting with the contract, we // must include any fragment we wish to use 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 (bool)", // Events "event Transfer(address indexed from, address indexed to, uint amount)" ]; // This can be an address or an ENS name const address = "0x4f3b15e4421902c09895fB12c8e0B8821134eA39"; // 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);

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)

See Meta-Class Filters for examples using events.

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.

await erc20.decimals(); // 18
erc20.balanceOf( owner [ , overrides ] ) Promise< BigNumber >

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

await erc20.balanceOf(signer.getAddress()) // { BigNumber: "100000000000000000000" }
erc20.symbol( [ overrides ] ) Promise< string >

Returns the symbol of the token.

await erc20.symbol(); // 'MyToken'
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.

// Before... formatUnits(await erc20_rw.balanceOf(signer.getAddress())); // '100.0' // Transfer 1.23 tokens to the ENS name "ricmoo.eth" tx = await erc20_rw.transfer("ricmoo.eth", parseUnits("1.23")); // { // chainId: 1337, // confirmations: 0, // data: '0xa9059cbb0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca0000000000000000000000000000000000000000000000001111d67bb1bb0000', // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // gasLimit: { BigNumber: "51558" }, // gasPrice: { BigNumber: "1" }, // hash: '0x6e0ed3f77f6188af90392579de28cfb35a60ef9b635d10080e78fb5cf3bdc7a4', // nonce: 2, // r: '0xaaf69c1abdb54a4d79876af372407b657c30018552b8cbb298b0d56759a42374', // s: '0x359f89f1a5ac45d7d30630fc401ff1530fcd02fb0781a5262f9a87ebb4648391', // to: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // type: null, // v: 2709, // value: { BigNumber: "0" }, // wait: [Function] // } // Wait for the transaction to be mined... await tx.wait(); // { // blockHash: '0x0b18b6d13e74aa8e9cbc394e61675e706cee142a07d87b8d691ac08cad0a6da1', // blockNumber: 46, // byzantium: true, // confirmations: 1, // contractAddress: null, // cumulativeGasUsed: { BigNumber: "51558" }, // events: [ // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // args: [ // '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // '0x5555763613a12D8F3e73be831DFf8598089d3dCa', // { BigNumber: "1230000000000000000" }, // amount: { BigNumber: "1230000000000000000" }, // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // to: '0x5555763613a12D8F3e73be831DFf8598089d3dCa' // ], // blockHash: '0x0b18b6d13e74aa8e9cbc394e61675e706cee142a07d87b8d691ac08cad0a6da1', // blockNumber: 46, // data: '0x0000000000000000000000000000000000000000000000001111d67bb1bb0000', // decode: [Function], // event: 'Transfer', // eventSignature: 'Transfer(address,address,uint256)', // getBlock: [Function], // getTransaction: [Function], // getTransactionReceipt: [Function], // logIndex: 0, // removeListener: [Function], // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78', // '0x0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca' // ], // transactionHash: '0x6e0ed3f77f6188af90392579de28cfb35a60ef9b635d10080e78fb5cf3bdc7a4', // transactionIndex: 0 // } // ], // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // gasUsed: { BigNumber: "51558" }, // logs: [ // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // blockHash: '0x0b18b6d13e74aa8e9cbc394e61675e706cee142a07d87b8d691ac08cad0a6da1', // blockNumber: 46, // data: '0x0000000000000000000000000000000000000000000000001111d67bb1bb0000', // logIndex: 0, // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78', // '0x0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca' // ], // transactionHash: '0x6e0ed3f77f6188af90392579de28cfb35a60ef9b635d10080e78fb5cf3bdc7a4', // transactionIndex: 0 // } // ], // logsBloom: '0x00000001000000000800000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000008000000000000000008000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000020000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000000000001000000000200000001000000000000000000000000000000', // status: 1, // to: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // transactionHash: '0x6e0ed3f77f6188af90392579de28cfb35a60ef9b635d10080e78fb5cf3bdc7a4', // transactionIndex: 0, // type: 0 // } // After! formatUnits(await erc20_rw.balanceOf(signer.getAddress())); // '98.77' formatUnits(await erc20_rw.balanceOf("ricmoo.eth")); // '1.23'
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.

// The signer has enough tokens to send, so true is returned await erc20_rw.callStatic.transfer("ricmoo.eth", parseUnits("1.23")); // true // A random address does not have enough tokens to // send, in which case the contract throws an error erc20_random = erc20_rw.connect(randomWallet); await erc20_random.callStatic.transfer("ricmoo.eth", parseUnits("1.23")); // [Error: call revert exception] { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // args: [ // 'ricmoo.eth', // { BigNumber: "1230000000000000000" } // ], // code: 'CALL_EXCEPTION', // errorArgs: [ // 'insufficient token balance' // ], // errorName: 'Error', // errorSignature: 'Error(string)', // method: 'transfer(address,uint256)', // reason: 'insufficient token balance', // transaction: { // data: '0xa9059cbb0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca0000000000000000000000000000000000000000000000001111d67bb1bb0000', // from: '0x7E914bC9f126643953ef1296492693146a7F6Cf5', // to: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39' // } // }
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.

await erc20_rw.estimateGas.transfer("ricmoo.eth", parseUnits("1.23")); // { BigNumber: "34458" }
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.

await erc20_rw.populateTransaction.transfer("ricmoo.eth", parseUnits("1.23")); // { // data: '0xa9059cbb0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca0000000000000000000000000000000000000000000000001111d67bb1bb0000', // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // to: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39' // }
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.

query filter *from* events
filterFrom = erc20.filters.Transfer(signer.address); // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78' // ] // } // Search for transfers *from* me in the last 10 blocks logsFrom = await erc20.queryFilter(filterFrom, -10, "latest"); // [ // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // args: [ // '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // '0x5555763613a12D8F3e73be831DFf8598089d3dCa', // { BigNumber: "1230000000000000000" }, // amount: { BigNumber: "1230000000000000000" }, // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // to: '0x5555763613a12D8F3e73be831DFf8598089d3dCa' // ], // blockHash: '0x0b18b6d13e74aa8e9cbc394e61675e706cee142a07d87b8d691ac08cad0a6da1', // blockNumber: 46, // data: '0x0000000000000000000000000000000000000000000000001111d67bb1bb0000', // decode: [Function], // event: 'Transfer', // eventSignature: 'Transfer(address,address,uint256)', // getBlock: [Function], // getTransaction: [Function], // getTransactionReceipt: [Function], // logIndex: 0, // removeListener: [Function], // removed: false, // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78', // '0x0000000000000000000000005555763613a12d8f3e73be831dff8598089d3dca' // ], // transactionHash: '0x6e0ed3f77f6188af90392579de28cfb35a60ef9b635d10080e78fb5cf3bdc7a4', // transactionIndex: 0 // } // ] // Note that the args providees the details of the event, each // parameters is available positionally, and since our ABI // included parameter names also by name logsFrom[0].args // [ // '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // '0x5555763613a12D8F3e73be831DFf8598089d3dCa', // { BigNumber: "1230000000000000000" }, // amount: { BigNumber: "1230000000000000000" }, // from: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // to: '0x5555763613a12D8F3e73be831DFf8598089d3dCa' // ]
query filter with *to* events
filterTo = erc20.filters.Transfer(null, signer.address); // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // null, // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78' // ] // } // Search for transfers *to* me in the last 10 blocks // Note: the contract transferred totalSupply tokens to us // when it was deployed in its constructor logsTo = await erc20.queryFilter(filterTo, -10, "latest"); // [ // { // address: '0x4f3b15e4421902c09895fB12c8e0B8821134eA39', // args: [ // '0x0000000000000000000000000000000000000000', // '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // { BigNumber: "100000000000000000000" }, // amount: { BigNumber: "100000000000000000000" }, // from: '0x0000000000000000000000000000000000000000', // to: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78' // ], // blockHash: '0x62fd00943531646ad700ee14a3e63c2ff553497439db486ebf04a1c5c624ae9b', // blockNumber: 45, // data: '0x0000000000000000000000000000000000000000000000056bc75e2d63100000', // decode: [Function], // event: 'Transfer', // eventSignature: 'Transfer(address,address,uint256)', // getBlock: [Function], // getTransaction: [Function], // getTransactionReceipt: [Function], // logIndex: 0, // removeListener: [Function], // removed: false, // topics: [ // '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef', // '0x0000000000000000000000000000000000000000000000000000000000000000', // '0x0000000000000000000000006fad877db7c9a8663aa23178f8effdf3187d2b78' // ], // transactionHash: '0xe05aa74209e5b928013952153dd231b42a8fdffb4bcc103e47c59e80d78a8629', // transactionIndex: 0 // } // ] // Note that the args providees the details of the event, each // parameters is available positionally, and since our ABI // included parameter names also by name logsTo[0].args // [ // '0x0000000000000000000000000000000000000000', // '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78', // { BigNumber: "100000000000000000000" }, // amount: { BigNumber: "100000000000000000000" }, // from: '0x0000000000000000000000000000000000000000', // to: '0x6FAD877dB7C9a8663aA23178F8eFfdF3187d2b78' // ]
listen for events
// Listen to incoming events from signer: erc20.on(filterFrom, (from, to, amount, event) => { // The `from` will always be the signer address }); // Listen to incoming events to signer: erc20.on(filterTo, (from, to, amount, event) => { // The `to` will always be the signer address }); // Listen to all Transfer events: erc20.on("Transfer", (from, to, amount, event) => { // ... });