Building an SDK v1.0.1-beta.13 – Typechain


The TypeChain project provides developers a tool to generate TypeScript typings for smart contracts they are interacting with. This gives all the usual benefits of Typing for example – flagging an error if you try to call a function on the smart contract that doesn’t exist.

In the SDK we were using the @balancer-labs/typechain package which aims to provide TypeChain bindings for the most commonly used Balancer contracts but we decided it would be better to remove this dependency and generate the bindings as needed. This enables us to remain up to date with new contracts (e.g. Relayers) without waiting for the package support.

Making The Changes

TypeChain is really pretty easy to use but we had to add a few additonal changes to the SDK.

To generate the typed wrapper TypeChain uses the Smart Contract ABIs. These were added in src/lib/abi. These can be found in the balancer-v2-monorepo or even from etherscan if the contract is already deployed/verified.

TypeChain will generate appropriate code for a given web3 library. In the SDK we use ethers.js so we need to make sure the @typechain/ethers-v5 package is added to our dev dependencies. (See the other available targets here)

CLI Command
To actually generate the files we need to run the typechain command and specifify the correct target, path to ABIs, and out path. For example:

typechain --target ethers-v5 --out-dir src/contracts './src/lib/abi/Vault.json'

Will target ethers and use the Vault ABI to generate the bindings in the src/contracts dir. You can see the full CLI docs here.

Its recommended that the generated file are not commited to the codebase so we add src/contracts/ to .gitignore. And in package.json a helper is added to scripts:

"typechain:generate": "npx typechain --target ethers-v5 --out-dir src/contracts './src/lib/abi/Vault.json' './src/lib/abi/WeightedPoolFactory.json' './src/lib/abi/BalancerHelpers.json' './src/lib/abi/LidoRelayer.json' './src/lib/abi/WeightedPool.json'"

and the CI is updated to call this command post install.

Updating the code
The last change to make was removing the old package and replacing any references to it. This is almost a direct replacement and just requires updating to use the path from the new contracts path. E.g.:

// Old
import { BalancerHelpers__factory } from "@balancer-labs/typechain";
// New
import { BalancerHelpers__factory } from '@/contracts/factories/BalancerHelpers__factory';

// Example of use
this.balancerHelpers = BalancerHelpers__factory.connect(

Example Of The Benefits

During the updates one of the benefits was highlighted. A previous example was incorrectly calling the queryExit function on the BalancerHelpers contract. This is a function that although it is used like a view it is actually a special case that requires it to be used with an eth_call (see here for more info). This led to a Type warning when trying to access the response. After correctly updating to use a callStatic the response typing matched the expected.

// Incorrect version
const response = await contracts.balancerHelpers.queryExit(...);
// Shows: Property 'amountsIn' does not exist on type 'ContractTransaction'.

// Correct version
const response = await contracts.balancerHelpers.callStatic.queryExit
const response: [BigNumber, BigNumber[]] & {
    bptOut: BigNumber;
    amountsIn: BigNumber[];

Photo by Kristian Strand on Unsplash

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