web3-eth-subscribe.rst

web3.eth.subscribe

The web3.eth.subscribe function lets you subscribe to specific events in the blockchain.

subscribe

web3.eth.subscribe(type [, options] [, callback]);

Parameters

1. String - The subscription, you want to subscribe to.
2. Mixed - (optional) Optional additional parameters, depending on the subscription type.
3. Function - (optional) Optional callback, returns an error object as first parameter and the result as second. Will be called for each incoming subscription, and the subscription itself as 3 parameter.

Returns

EventEmitter - A Subscription instance

• subscription.id: The subscription id, used to identify and unsubscribing the subscription.
• subscription.subscribe([callback]): Can be used to re-subscribe with the same parameters.
• subscription.unsubscribe([callback]): Unsubscribes the subscription and returns TRUE in the callback if successfull.
• subscription.arguments: The subscription arguments, used when re-subscribing.
• on("data") returns Object: Fires on each incoming log with the log object as argument.
• on("changed") returns Object: Fires on each log which was removed from the blockchain. The log will have the additional property "removed: true".
• on("error") returns Object: Fires when an error in the subscription occurs.
• on("connected") returns String: Fires once after the subscription successfully connected. Returns the subscription id.

Notification returns

• Mixed - depends on the subscription, see the different subscriptions for more.

Example

var subscription = web3.eth.subscribe('logs', {
    address: '0x123456..',
    topics: ['0x12345...']
}, function(error, result){
    if (!error)
        console.log(result);
});

// unsubscribes the subscription
subscription.unsubscribe(function(error, success){
    if(success)
        console.log('Successfully unsubscribed!');
});

---

clearSubscriptions

web3.eth.clearSubscriptions()

Resets subscriptions.

Note

This will not reset subscriptions from other packages like web3-shh, as they use their own requestManager.

Parameters

1. Boolean: If true it keeps the "syncing" subscription.

Returns

Boolean

Example

web3.eth.subscribe('logs', {} ,function(){ ... });

...

web3.eth.clearSubscriptions();

---

subscribe("pendingTransactions")

web3.eth.subscribe('pendingTransactions' [, callback]);

Subscribes to incoming pending transactions.

Parameters

1. String - "pendingTransactions", the type of the subscription.
2. Function - (optional) Optional callback, returns an error object as first parameter and the result as second. Will be called for each incoming subscription.

Returns

EventEmitter: An :ref:`subscription instance <eth-subscription-return>` as an event emitter with the following events:

• "data" returns String: Fires on each incoming pending transaction and returns the transaction hash.
• "error" returns Object: Fires when an error in the subscription occurs.

Notification returns

1. Object|Null - First parameter is an error object if the subscription failed.
2. String - Second parameter is the transaction hash.

Example

var subscription = web3.eth.subscribe('pendingTransactions', function(error, result){
    if (!error)
        console.log(result);
})
.on("data", function(transaction){
    console.log(transaction);
});

// unsubscribes the subscription
subscription.unsubscribe(function(error, success){
    if(success)
        console.log('Successfully unsubscribed!');
});

---

subscribe("newBlockHeaders")

web3.eth.subscribe('newBlockHeaders' [, callback]);

Subscribes to incoming block headers. This can be used as timer to check for changes on the blockchain.

Parameters

1. String - "newBlockHeaders", the type of the subscription.
2. Function - (optional) Optional callback, returns an error object as first parameter and the result as second. Will be called for each incoming subscription.

Returns

EventEmitter: An :ref:`subscription instance <eth-subscription-return>` as an event emitter with the following events:

• "data" returns Object: Fires on each incoming block header.
• "error" returns Object: Fires when an error in the subscription occurs.
• "connected" returns Number: Fires once after the subscription successfully connected. Returns the subscription id.

The structure of a returned block header is as follows:

• number - Number: The block number. null when its pending block.
• hash 32 Bytes - String: Hash of the block. null when its pending block.
• parentHash 32 Bytes - String: Hash of the parent block.
• nonce 8 Bytes - String: Hash of the generated proof-of-work. null when its pending block.
• sha3Uncles 32 Bytes - String: SHA3 of the uncles data in the block.
• logsBloom 256 Bytes - String: The bloom filter for the logs of the block. null when its pending block.
• transactionsRoot 32 Bytes - String: The root of the transaction trie of the block
• stateRoot 32 Bytes - String: The root of the final state trie of the block.
• receiptsRoot 32 Bytes - String: The root of the receipts.
• miner - String: The address of the beneficiary to whom the mining rewards were given.
• extraData - String: The "extra data" field of this block.
• gasLimit - Number: The maximum gas allowed in this block.
• gasUsed - Number: The total used gas by all transactions in this block.
• timestamp - Number: The unix timestamp for when the block was collated.

Notification returns

1. Object|Null - First parameter is an error object if the subscription failed.
2. Object - The block header object like above.

Example

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

        return;
    }

    console.error(error);
})
.on("connected", function(subscriptionId){
    console.log(subscriptionId);
})
.on("data", function(blockHeader){
    console.log(blockHeader);
})
.on("error", console.error);

// unsubscribes the subscription
subscription.unsubscribe(function(error, success){
    if (success) {
        console.log('Successfully unsubscribed!');
    }
});

---

subscribe("syncing")

web3.eth.subscribe('syncing' [, callback]);

Subscribe to syncing events. This will return an object when the node is syncing and when its finished syncing will return FALSE.

Parameters

1. String - "syncing", the type of the subscription.
2. Function - (optional) Optional callback, returns an error object as first parameter and the result as second. Will be called for each incoming subscription.

Returns

EventEmitter: An :ref:`subscription instance <eth-subscription-return>` as an event emitter with the following events:

• "data" returns Object: Fires on each incoming sync object as argument.
• "changed" returns Object: Fires when the synchronisation is started with true and when finished with false.
• "error" returns Object: Fires when an error in the subscription occurs.

For the structure of a returned event Object see :ref:`web3.eth.isSyncing return values <eth-issyncing-return>`.

Notification returns

1. Object|Null - First parameter is an error object if the subscription failed.
2. Object|Boolean - The syncing object, when started it will return true once or when finished it will return false once.

Example

var subscription = web3.eth.subscribe('syncing', function(error, sync){
    if (!error)
        console.log(sync);
})
.on("data", function(sync){
    // show some syncing stats
})
.on("changed", function(isSyncing){
    if(isSyncing) {
        // stop app operation
    } else {
        // regain app operation
    }
});

// unsubscribes the subscription
subscription.unsubscribe(function(error, success){
    if(success)
        console.log('Successfully unsubscribed!');
});

---

subscribe("logs")

web3.eth.subscribe('logs', options [, callback]);

Subscribes to incoming logs, filtered by the given options. If a valid numerical fromBlock options property is set, Web3 will retrieve logs beginning from this point, backfilling the response as necessary.

Parameters

1. "logs" - String, the type of the subscription.
2. Object - The subscription options

• fromBlock - Number: The number of the earliest block. By default null.
• address - String|Array: An address or a list of addresses to only get logs from particular account(s).
• topics - Array: An array of values which must each appear in the log entries. The order is important, if you want to leave topics out use null, e.g. [null, '0x00...']. You can also pass another array for each topic with options for that topic e.g. [null, ['option1', 'option2']]

3. callback - Function: (optional) Optional callback, returns an error object as first parameter and the result as second. Will be called for each incoming subscription.

Returns

EventEmitter: An :ref:`subscription instance <eth-subscription-return>` as an event emitter with the following events:

• "data" returns Object: Fires on each incoming log with the log object as argument.
• "changed" returns Object: Fires on each log which was removed from the blockchain. The log will have the additional property "removed: true".
• "error" returns Object: Fires when an error in the subscription occurs.
• "connected" returns Number: Fires once after the subscription successfully connected. Returns the subscription id.

For the structure of a returned event Object see :ref:`web3.eth.getPastEvents return values <eth-getpastlogs-return>`.

Notification returns

1. Object|Null - First parameter is an error object if the subscription failed.
2. Object - The log object like in :ref:`web3.eth.getPastEvents return values <eth-getpastlogs-return>`.

Example

var subscription = web3.eth.subscribe('logs', {
    address: '0x123456..',
    topics: ['0x12345...']
}, function(error, result){
    if (!error)
        console.log(result);
})
.on("connected", function(subscriptionId){
    console.log(subscriptionId);
})
.on("data", function(log){
    console.log(log);
})
.on("changed", function(log){
});

// unsubscribes the subscription
subscription.unsubscribe(function(error, success){
    if(success)
        console.log('Successfully unsubscribed!');
});