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content/Blockchain/Common typescript examples.md

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+- Simple #dapp example for tests: [https://metamask.github.io/test-dapp/](https://metamask.github.io/test-dapp/)
+- Interaction with smart contracts described in [Smart contracts](Smart%20contracts.md)
+
+## Connecting to node
+
+If #Metamask extension installed, `Web3.givenProvider` is available in global window. You can use [Infura](https://infura.io) or your node instead:
+
+```typescript
+import Web3 from 'web3';
+
+// URL of your node
+const PROVIDER_URL = 'https://...';
+
+export const web3 = new Web3(Web3.givenProvider || PROVIDER_URL);
+``` 
+
+## Getting wallet balance
+
+```typescript
+const getBalance = async (address: string) => {
+    return await web3.eth.getBalance(address);
+}
+```
+
+## Getting wallet address
+
+```typescript
+// first we need to authorize
+const authorize = async () => {
+  await web3.currentProvider.request({ method: 'eth_requestAccounts' });
+}
+
+// then we can get wallet address
+const getCurrentAddressUser = () => {
+    return web3.currentProvider.selectedAddress;
+}
+```
+
+## Sending transaction
+
+Sending `value` tokens with `memo` as value:
+
+```typescript
+
+const transfer = async ({ 
+  from, 
+  to, 
+  value, 
+  memo, 
+  privateKey, 
+  gasLimit = 44000 
+}) => {
+    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.
+
+```typescript
+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
+
+```typescript
+import { web3 } from '.';
+
+web3.currentProvider.on('accountsChanged', callback);
+```
+
+
+## Watching network change
+
+```typescript
+ethereum.on('chainChanged', handler: (chainId: string) => void);
+```
+
+## Adding custom token to wallet
+
+```typescript
+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:
+
+```typescript
+const getChainID = async () => {
+    return ethereum.request({ method: 'eth_chainId' })
+}
+```
+
+Asking wallet to change current network:
+
+```typescript
+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
+}
+```
+