# Common Typescript Examples

- Simple #dapp example for tests: https://metamask.github.io/test-dapp/
- Interaction with smart contracts described in Smart contracts

## Connecting to node

If #Metamask extension installed, `Web3.givenProvider` is available in global window. You can use Infura or your node instead:

    import Web3 from 'web3';// URL of your nodeconst PROVIDER_URL = 'https://...';export const web3 = new Web3(Web3.givenProvider || PROVIDER_URL);

## Getting wallet balance

    const getBalance = async (address: string) => {    return await web3.eth.getBalance(address);}

## Getting wallet address

    // first we need to authorizeconst authorize = async () => {  await web3.currentProvider.request({ method: 'eth_requestAccounts' });}// then we can get wallet addressconst getCurrentAddressUser = () => {    return web3.currentProvider.selectedAddress;}

## Sending transaction

Sending `value` tokens with `memo` as value:

    const transfer = async ({   from,   to,   value,   memo,   privateKey,   gasLimit = }) => {    const nonce = await web3.eth.getTransactionCount(from);    const gasPrice = await web3.eth.getGasPrice();        const rawTx = {      from,      to,      value: web3.utils.toHex(Web3.utils.toWei(value, 'ether')),      gasLimit: web3.utils.toHex(gasLimit),      gasPrice: web3.utils.toHex(gasPrice),      nonce: web3.utils.toHex(nonce),      data: memo,    };        const privateKeyBuffer = EthUtil.toBuffer(privateKey);        const tx = new Transaction(rawTx);        tx.sign(privateKeyBuffer);    const serializedTx = tx.serialize();        return this.web3.eth.sendSignedTransaction(      `0x${serializedTx.toString('hex')}`    );}

## Estimating transaction FEE

Useful to get fixed amount of tokens from user with pre-estimated fee.

    import { web3 } from '.';const estimateFee = async ({    from,    to,    value,    memo,}) => {    const gasPrice = await web3.eth.getGasPrice();    const gasLimit = await web3.eth.estimateGas({      from,      to,      value: web3.utils.toHex(web3.utils.toWei(value, 'ether')),      data: web3.utils.asciiToHex(memo),    }).call();        return web3.utils.fromWei(      BigInt(gasPrice.toString())        .multiply(BigInt(gasLimit.toString()))        .toString()    );}

## Subscribing to wallet address change

    import { web3 } from '.';web3.currentProvider.on('accountsChanged', callback);

## Watching network change

    ethereum.on('chainChanged', handler:

## Adding custom token to wallet

    window.ethereum  .request({    method: 'wallet_watchAsset',    params: {      type: 'ERC20',      options: {        address: '0xb60e8dd61c5d32be8058bb8eb970870f07233155',        symbol: 'FOO',        decimals: 18,        image: 'https://foo.io/token-image.svg',      },    },  })  .then((success) => {    if (success) {      console.log('FOO successfully added to wallet!')    } else {      throw new Error('Something went wrong.')    }  })  .catch(console.error)

## Changing network to custom

Checking current chainId:

    const getChainID = async () => {    return ethereum.request({ method: 'eth_chainId' })}

Asking wallet to change current network:

    try {  await window.ethereum.request({    method: 'wallet_switchEthereumChain',    params: [{ chainId: '0x03' }], // ropsten chainID (3) in hex  });} catch (switchError) {  // This error code indicates that the chain has not been added to MetaMask.  if (error.code === 4902) {    try {      await window.ethereum.request({        method: 'wallet_addEthereumChain',        params: [{           chainId: '0x03', // ropsten chainID (3) in hex          chainName: 'Ropsten Test Network',           nativeCurrency: {               name: 'ETH',              symbol: 'ETH',              decimals: 18          },           rpcUrls: ['https://ropsten.infura.io/v3/9aa3d95b3bc440fa88ea12eaa4456161'],           blockExplorerUrls: ['https://ropsten.etherscan.io']         }] ,      });    } catch (addError) {      // handle "add" error    }  }  // handle other "switch" errors}