---
description: >-
A tool for generating type-safe TypeScript classes for runtime events and
extrinsic calls
---

Hydra Typegen

Motivation

Hydra Typegen is a code generation tool for creating Typescript types for substrate events and extrinsics. Its primary use-case is to provide type-safe interfaces for Hydra mappings. For example, once a typescript class for the Balances.Transfer event is generated, a mapping can look like

``typescript
export async function balancesTransfer({
event
}: EventContext & StoreContext ) {
const [from, to, value] = new Balances.TransferEvent(event).params
const { dest, value } = new Balances.TransferCall(event).args
...
}
`

Quickstart

A minimal example for generating classes for the Balances.Transfer and Treasury.Deposit events in Kusama:

`bash
hydra-typegen typegen --metadata wss://kusama-rpc.polkadot.io Balances.Transfer
`

It is also possible to run hydra-typegen against a YAML config file

`bash
hydra-typegen typegen typegen.yml --debug
`

Typegen config

Typegen config file has the following structure:





















































Field	Type	Description
metadata.source	string	Where typegen will source node metadata. Can either be a ws endpoint or a path to a static json file with the same content as the result of state_getMetadata RPC call
metadata.blockHash	string	(optional) If metadata.source is a WS endpoint, hash of the block from which metadata will be sourced, in hex format
events	[string]	A list of events for which TS classes will be generated, in the format <section>.<name>
calls	[string]	A list of extrinsics (calls) for which TS classes will be generated, in the format <section>.<method>
outDir	string	Root directory for the generated classes
strict	boolean	Default: false If true, the event/extrinsic constructor throws an error if the raw data does not match the format in metadata (e.g. due to a runtime upgrade).
customTypes.lib	string	(optional) Import path for custom types that will be used in the generated sources
customTypes.typedefs	string	(optional) Path to a JSON file with custom type definitions, as expected by polkadot.js createApi method

The config file typegen.yml can look like this:

`text


Typegen will pull the metadata from Kusama at block with the given hash

metadata:
source: wss://kusama-rpc.polkadot.io
blockHash: '0x45eb7ddd324361adadd4f8cfafadbfb7e0a26393a70a70e5bee6204fc46af62e'

events and calls for which the typescript types will be generated

events:
- Balances.Transfer
calls:
- Balances.transfer
outDir: ./generated
`

Custom types

Hydra Typegen supports custom substrate types via the --typedefs flag. The provided .json file should include type definitions for the arguments and parameters of the events and extrinsics to be generated. The type definitions file is copied to the generated sources.

In the config file, place the definition into the customTypes section. It assumes that all the custom runtime types are already available for import from a library, so that e.g. the generated import statement

`text
import { MyCustomRuntimeClass } from 'my/types/library'
`

is correctly resolved.

`text
...
customTypes:
lib: 'my/types/library',
typedefs: my-types-json,
`

Note, that when used in the mappings, the library with custom types \(here my/types/library\) must be added as a dependency for the mappings module in mappings/package.json

Commands

* [hydra-typegen help [COMMAND]](hydra-typegen.md#hydra-typegen-help-command)
* [hydra-typegen typegen [CONFIG]](hydra-typegen.md#hydra-typegen-typegen-config)

hydra-typegen help [COMMAND]

display help for hydra-typegen

`text
USAGE
$ hydra-typegen help [COMMAND]

ARGUMENTS
COMMAND command to show help for

OPTIONS
--all see all commands in CLI
`

_See code:_ _@oclif/plugin-help_

hydra-typegen typegen [CONFIG]

Generate Typescript classes for the Substrate events

`text
USAGE
$ hydra-typegen typegen [CONFIG]

ARGUMENTS
CONFIG Path to YML config file. Overrides the flag options

OPTIONS
-c, --calls=calls Comma-separated list of substrate calls in the format .
-d, --debug Output debug info
-e, --events=events Comma-separated list of substrate events in the formation .

-h, --blockHash=blockHash Hash of the block from which the metadata will be fetched. Only applied if metadata is
pulled via an RPC call

-i, --typelib=typelib A JavaScript module from which the custom types should be imported, e.g.
'@joystream/types/augment'

-m, --metadata=metadata [default: metadata.json] Chain metadata source. If starts with ws:// or wss:// the metadata
is pulled by an RPC call to the provided endpoint. Otherwise a relative path to a json file
matching the RPC call response is expected

-o, --outDir=outDir [default: generated/types] A relative path the root folder where the generated files will
be generated

-s, --[no-]strict Strict mode. If on, the generated code throws an error if the input event argument types
don't much the metadata definiton

-t, --typedefs=typedefs A relative path to a file with JSON definitions for custom types used by the chain
``

_See code:_ _src/commands/typegen/index.ts_

A full sample Hydra project can be found here