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Make sure you have set up Matic.js by following the get started guide.
The POSClient interacts with the POS bridge. 
import { POSClient,use } from "@maticnetwork/maticjs"
import { Web3ClientPlugin } from '@maticnetwork/maticjs-web3'
import HDWalletProvider from "@truffle/hdwallet-provider"

// install web3 plugin
use(Web3ClientPlugin);

const posClient = new POSClient();
await posClient.init({
    network: 'testnet',
    version: 'amoy',
    parent: {
      provider: new HDWalletProvider(privateKey, mainRPC),
      defaultConfig: {
        from : fromAddress
      }
    },
    child: {
      provider: new HDWalletProvider(privateKey, childRPC),
      defaultConfig: {
        from : fromAddress
      }
    }
});
After the POSClient is initiated, we can work with token types, such as erc20, erc721 etc.

​
ERC20

​
Create ERC20 child token

const erc20ChildToken = posClient.erc20(<token address>);

​
Create ERC20 parent token

const erc20ParentToken = posClient.erc20(<token address>, true);
Once erc20 is initaited, you can call various methods that are available, like - getBalance, approve, deposit , withdraw etc.

​
getBalance

const balance = await erc20ChildToken.getBalance(<userAddress>)
console.log('balance', balance)

​
approve

// approve amount 10 on parent token
const approveResult = await erc20ParentToken.approve(10);

// get transaction hash
const txHash = await approveResult.getTransactionHash();

// get transaction receipt
const txReceipt = await approveResult.getReceipt();
As you can see, with its simple APIs maticjs makes it very easy to interact with maticjs bridge. Once the POSClient is initiated, you can interact with all available APIs.