web3-contract-basic-interaction-async-await.asciidoc

web3.js contract basic interaction (async/await) as if they were synchronous functions

Description

This script is for educational use and is based on web3@1.0.0-beta.29 web3.js version.

It should be see as an introduction to web3.js.

The web3.js library is a collection of modules which contain specific functionality for the Ethereum ecosystem.

The web3.js object is an umbrella package to house all Ethereum related modules.

This is the Ethereum compatible JavaScript API which implements the Generic JSON RPC spec.

It run in a blocked (async/await) mode to move the reader away from the hell of the Promises as long as their version of nodejs or javascript allows it.

The most remarkable thing about this script is that you don’t need to run your own local node to use it, because it use the Infura services.

Prepare the environment

To see this script in action you should follow this simple steps.

Check you have a valid npm version

$ npm -v
5.6.0

If you don’t have already done, initialize your package

$ npm init

Install basic dependences

npm i web3
npm i node-rest-client-promise

This will update your package.json cofiguracion file with your new dependences.

Node.js script execution

Execution example;

code/web3js/./web3-contract-basic-interaction-async-await.js

What the hell this script do?

This script try to introduce to the basic use of web3.js using async/await (less Code), as if they were synchronous functions that return values ​​directly instead of promises.

web3 provider

We use web3.js Web3 object to obtain a basic web3 provider.

var web3 = new Web3(infura_host);

Let’s do some basic interactions at web3 level

Let’s get the Protocol Version.

var protocolVersion = await web3.eth.getProtocolVersion();

Now I’m curious about the current gas price.

var gasPrice = await web3.eth.getGasPrice();

And, Whats the last mined block in my chain?

var blockNumber = await web3.eth.getBlockNumber();

Now let’s dive into some basics actions with a contract

We will use the contract at; https://kovan.etherscan.io/address/0xd0a1e359811322d97991e03f863a0c30c2cf029c#code

First things first, let’s initialize our contract address.

var our_contract_address = "0xd0A1E359811322d97991E03f863a0C30C2cF029C";

Let’s get its balance.

var balance = await web3.eth.getBalance(our_contract_address);

Now let’s get its byte code.

var code = await web3.eth.getCode(our_contract_address);

Now we are going to deal with the contract

We prepare our environment to interact with the Etherescan explorer API.

Let’s initialize our contract url in the Etherescan explorer API for the Kovan chain.

var etherescan_url = `http://kovan.etherscan.io/api?module=contract&action=getabi&address=${our_contract_address}`

And now get a rest client to operate with.

var client = require('node-rest-client-promise').Client();

Let’s get the client response.

var etherescan_response = await client.getPromise(etherescan_url)

Now we get here our contract ABI from the client response (from the Etherescan explorer).

our_contract_abi = JSON.parse(etherescan_response.data.result);

Let’s instantiate our contract object

var our_contract = await new web3.eth.Contract(our_contract_abi, our_contract_address);

Let’s see our contract address.

console.log(`Our Contract address:  ${our_contract._address}`);

or in this other way.

console.log(`Our Contract address in other way:  ${our_contract.options.address}`);

Now our contract abi.

console.log("Our contract abi: " + JSON.stringify(our_contract.options.jsonInterface));

This is turning more interesting, let’s see what’s going on with our contract

Now let’s get our contract total supply.

var totalSupply = await our_contract.methods.totalSupply().call();

Now let’s get our contract public variable name.

var name = await our_contract.methods.name().call();

Now let’s get our contract public variable symbol.

var symbol = await our_contract.methods.symbol().call();