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Light client for Polkadot and Substrate-based chains

This JavaScript library provides a light client for the Polkadot blockchain and for chains built using the Substrate blockchain framework.

It is an “actual” light client, in the sense that it is byzantine-resilient. It does not rely on the presence of an RPC server, but directly connects to the full nodes of the network.


import * as smoldot from 'smoldot';

// Load a string chain specification.
const chainSpec = fs.readFileSync('./westend.json', 'utf8');

// A single client can be used to initialize multiple chains.
const client = smoldot.start();

const chain = await client.addChain({ chainSpec });


// Wait for a JSON-RPC response to come back. This is typically done in a loop in the background.
const jsonRpcResponse = await chain.nextJsonRpcResponse();

// Later:
// chain.remove();


The first thing to do is to initialize the client with the start function.

Once initialized, the client can be used to connect to one or more chains. Use addChain to add a new chain that the client must be connected to. addChain must be passed the specification of the chain (commonly known as “chain spec”).

The addChain function returns a Promise that yields a chain once the chain specification has been successfully parsed and basic initialization is finished, but before Internet connections are opened towards the chains.

In order to de-initialize a chain, call chain.remove(). Any function called afterwards on this chain will throw an exception. In order to de-initialize a client, call client.terminate(). Any function called afterwards on any of the chains of the client will throw an exception.

After having obtained a chain, use sendJsonRpc to send a JSON-RPC request towards the node. The function accepts as parameter a string request. See the specification of the JSON-RPC protocol, and the list of requests that smoldot is capable of serving. Smoldot also has experimental support for an extra (still experimental at the time of writing of this comment) set of JSON-RPC functions found here.

If the request is well formatted, the client will generate a response. This response can be pulled using the nextJsonRpcResponse asynchronous function. Calling this function waits until a response is available and returns it.

If the request is a subscription, the notifications will also be sent back using the same mechanism and can be pulled using nextJsonRpcResponse.

If the chain specification passed to addChain is a parachain, then the list of potential relay chains must be passed as parameter to addChain as well. In situations where the chain specifications passed to addChain are not trusted, it is important for security reasons to not establish a parachain-relay-chain link between two chains that aren’t part of the same “trust sandbox”.