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Development of smart contracts and creating tokens on Ethereum, buying and selling ETH, other consulting.

Set up web3.js to use the Ethereum blockchain in JavaScript

In this tutorial, we’ll see how to get started with web3.js to interact with the Ethereum blockchain. Web3.js can be used both in frontends and backends to read data from the blockchain or make transactions and even deploy smart contracts.

The first step is to include web3.js in your project. To use it in a web page, you can import the library directly using a CDN like JSDeliver.

1<script src="https://cdn.jsdelivr.net/npm/web3@latest/dist/web3.min.js"></script>

If you prefer installing the library to use in your backend or a frontend project that uses build you can install it using npm:

npm install web3 --save

Then to import Web3.js into a Node.js script or Browserify front-end project, you can use the following line of JavaScript:

1const Web3 = require("web3")
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Now that we included the library in the project we need to initialize it. If your project needs to be able to communicate with the blockchain. Most Ethereum librairies communicate with a node through RPC calls. To initiate our web3 provider, we’ll instantiate a Web3 instance passing as the constructor the URL of the provider. If you have a node or ganache instance running on your computer it will look like this:

1const web3 = new Web3("http://localhost:8545")
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If you’d like to directly access a hosted node you can use Infura or the free ones provided by Cloudfare:

1const web3 = new Web3("https://cloudflare-eth.com")
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To test that we correctly configured our web3 instance, we’ll try to retrieve the latest block number by using the getBlockNumber function. This function accepts a callback as parameter and return the block number as an integer.

1var Web3 = require("web3")
2const web3 = new Web3("https://cloudflare-eth.com")
4web3.eth.getBlockNumber(function (error, result) {
5 console.log(result)
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If you execute this program, it will simply print the latest block number: the top of the blockchain. You can also use await/async function calls to avoid nesting callbacks in your code:

1async function getBlockNumber() {
2 const latestBlockNumber = await web3.eth.getBlockNumber()
3 console.log(latestBlockNumber)
4 return latestBlockNumber
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You can see all the functions that are available on the web3 instance in the official web3 documentation.

Most of Web3 libraries are asynchronous because in the background the library makes JSON RPC calls to the node which send backs the result.

If you are working in the browser, some wallets directly inject a web3 instance and you should try to use it whenever possible especially if you plan to interact with the user’s Ethereum address to make transactions.

Here is the snippet to detect if a Metamask wallet is available and try to enable it if it is. It will later allow you to read the user’s balance and enable them to validate transactions you’d like to make them do on the Ethereum blockchain:

1if (window.ethereum != null) {
2 state.web3 = new Web3(window.ethereum)
3 try {
4 // Request account access if needed
5 await window.ethereum.enable()
6 // Acccounts now exposed
7 } catch (error) {
8 // User denied account access...
9 }
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Alternatives to web3.js like Ethers.js do exist but we’ll focus all our JavaScript tutorials on web3.js as it is the official library to interact with Ethereum in the browser. In the next tutorial we’ll see how to easily listen to new incoming blocks on the blockchain and see what they contains.

Paul Wackerow
Last edit: @wackerow, 3. november 2020
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