web3.eth.subscribe Migration Guide

Breaking Changes

Subscribing to events

You subscribe to blockchain events using the web3.eth.subscribe API.

However, in web3.js version 1.x, for example, you could subscribe to the newBlockHeaders event, in one step, with the following code snippet:

var subscription = web3.eth.subscribe('newBlockHeaders', function (error, result) {
	if (!error) console.log(result);
});

But, in web3.js Version 4.x, the function signature has changed for web3.eth.subscribe. In addition, the way you get notified for data and error has also changed. It is now in 2 steps: First you subscribe and then you listen to events. Here's an example of how you would subscribe to the same newBlockHeaders event in web3.js version 4.x:

// in 4.x
const subscription = await web3.eth.subscribe('newHeads');

// note that in version 4.x the way you get notified for `data` and `error` has changed
subscription.on('data', async blockhead => {
	console.log('New block header: ', blockhead);
});
subscription.on('error', error =>
	console.log('Error when subscribing to New block header: ', error),
);

Differences

In summary, the differences you need to be aware of when subscribing to blockchain events in web3.js version 4.x are:

• The subscribe function signature has changed:
  • It does not accept a callback function.
  • It returns a subscription object that you can use to listen to data and error events.
• You should now use the on, or once, method on the newly returned subscription object to listen to data and error events, instead of passing a callback function directly.
• You can have multiple event listeners, if you have, for example multiple on calls. And you can get the number of listeners in you code by calling listenerCount(event_name) or get the listeners with listeners(event_name).

Keep in mind that these differences apply to all blockchain event subscriptions, not just to the newBlockHeaders event.

New Block Headers event

In 1.x, web3.eth.subscribe('newBlockHeaders') was used to subscribe to new block headers.

In 4.x, either web3.eth.subscribe('newHeads') or web3.eth.subscribe('newBlockHeaders') can be used to subscribe to new block headers.

web3.eth.clearSubscriptions

In 1.x, web3.eth.clearSubscriptions returns true on success.

In 4.x, web3.eth.clearSubscriptions returns an Array of subscription ids.

// in 1.x
var subscription = web3.eth.subscribe('newBlockHeaders', function (error, result) {
	if (!error) console.log(result);
});
web3.eth.clearSubscriptions(function (error, success) {
	console.log(success); // true
});

// in 4.x
const subscription = await web3.eth.subscribe('newHeads');

// note that in version 4.x the way you get notified for `data` and `error` has changed
newBlocksSubscription.on('data', async blockhead => {
	console.log('New block header: ', blockhead);
});
newBlocksSubscription.on('error', error =>
	console.log('Error when subscribing to New block header: ', error),
);

const ids = await web3.eth.clearSubscriptions();
console.log(ids); // [...] An array of subscription ids that were cleared

// note that you can unsubscribe from a specific subscription by calling unsubscribe()
//	on that subscription object: `await subscription.unsubscribe();` and this would return void if succeeded.