Polkadot API Patterns

polkadot-api Patterns

Essential patterns for developing with polkadot-api (PAPI).

Critical Rule: Package Imports

NEVER use @polkadot/api (PJS) - ONLY use polkadot-api (PAPI).

Correct Imports

// Core PAPI
import { Binary, createClient, type Transaction, type TypedApi } from 'polkadot-api'

// WebSocket
import { getWsProvider } from 'polkadot-api/ws-provider'

// Descriptors (generated)
import { MultiAddress, type qfn } from '@polkadot-api/descriptors'

// Utils
import { type PolkadotSigner } from '@polkadot-api/utils'

Forbidden Imports

// ❌ NEVER import these
import { ApiPromise } from '@polkadot/api'
import { WsProvider } from '@polkadot/rpc-provider'

Why: PJS and PAPI are different, incompatible libraries. PJS is maintenance mode, PAPI is the future.

Client Setup

Create client once, reuse TypedApi:

import { createClient } from 'polkadot-api'
import { getWsProvider } from 'polkadot-api/ws-provider'
import { qfn as chain } from '@polkadot-api/descriptors'

// Provider
const provider = getWsProvider('wss://test.qfnetwork.xyz')

// Client (connection manager)
const client = createClient(provider)

// TypedApi (typed interface)
const api = client.getTypedApi(chain)

// Type for signatures
type QfnApi = TypedApi<typeof qfn>

Access: api.tx.* api.query.* api.constants.*

Type Wrappers

MultiAddress.Id() for Addresses

import { MultiAddress } from '@polkadot-api/descriptors'

// ✅ Always wrap addresses
const tx = api.tx.Assets.transfer({
  id: assetId,
  target: MultiAddress.Id(recipientAddress),
  amount: 1000n
})

// ❌ Raw strings fail
target: recipientAddress  // Type error!

Binary.fromText() for Strings

import { Binary } from 'polkadot-api'

// ✅ Wrap strings for chain
const tx = api.tx.Assets.set_metadata({
  id: assetId,
  name: Binary.fromText("Token Name"),
  symbol: Binary.fromText("SYMB"),
  decimals: 12
})

// ❌ Raw strings fail
name: "Token Name"  // Type error!

Transaction Patterns

Basic Transaction

// Build
const tx = api.tx.Balances.transfer_keep_alive({
  dest: MultiAddress.Id(recipientAddress),
  value: 1_000_000_000_000n
})

// Execute
const observable = tx.signSubmitAndWatch(polkadotSigner)

Batch Transactions

All-or-nothing atomicity with Utility.batch_all:

import { Binary, type TxCallData } from 'polkadot-api'

// Build calls
const createCall = api.tx.Assets.create({
  id: assetId,
  admin: MultiAddress.Id(signerAddress),
  min_balance: minBalance
}).decodedCall  // ← Key: use .decodedCall

const metadataCall = api.tx.Assets.set_metadata({
  id: assetId,
  name: Binary.fromText("Token"),
  symbol: Binary.fromText("TKN"),
  decimals: 12
}).decodedCall

// Batch
const calls: TxCallData[] = [createCall, metadataCall]
const batchTx = api.tx.Utility.batch_all({ calls })

Pattern: Individual tx → .decodedCall → Array → Utility.batch_all({ calls })

Observable Lifecycle

import type { TxBroadcastEvent } from 'polkadot-api'

observable.subscribe({
  next: (event: TxBroadcastEvent) => {
    if (event.type === 'broadcasted') {
      console.log('Broadcasted')
    }
    if (event.type === 'txBestBlocksState' && event.found) {
      console.log('In block')
    }
    if (event.type === 'finalized') {
      console.log('Finalized:', event.block.hash)
    }
  },
  error: (err) => console.error('Transaction error:', err),
  complete: () => console.log('Complete')
})

Events: broadcasted → txBestBlocksState → finalized

Query Patterns

Single Value

// Current value
const balance = await api.query.System.Account.getValue(address)

// At block
const balanceAt = await api.query.System.Account.getValue(address, { at: blockHash })

Multiple Entries

// All entries
const assets = await api.query.Assets.Asset.getEntries()

assets.forEach(({ keyArgs, value }) => {
  const [assetId] = keyArgs
  console.log(`Asset ${assetId}:`, value)
})

// With pagination
const page = await api.query.Assets.Asset.getEntries({ at: 'best', pageSize: 100 })

Multi-key Query

// Multiple keys at once
const addresses = [addr1, addr2, addr3]
const balances = await api.query.System.Account.getValues(addresses)

Watch Changes

// Subscribe to updates
const unsubscribe = api.query.System.Account.watchValue(
  address,
  (accountInfo) => console.log('Balance:', accountInfo.data.free)
)

// Clean up
unsubscribe()

Constants

// Access chain constants
const existentialDeposit = api.constants.Balances.ExistentialDeposit()
const maxAssets = api.constants.Assets.MaxAssets()

Error Handling

See references/error-handling.md for complete error handling patterns:

• Observable errors (before finalization)
• Pallet errors (runtime errors)
• Query errors
• Connection errors
• User-friendly error messages

Common Mistakes

Missing Type Wrappers

// ❌ Wrong
api.tx.Assets.transfer({
  target: recipientAddress,  // Missing MultiAddress.Id()
})

// ✅ Correct
api.tx.Assets.transfer({
  target: MultiAddress.Id(recipientAddress),
})

Not Using .decodedCall for Batches

// ❌ Wrong - missing .decodedCall
const calls = [
  api.tx.Assets.create({ ... }),
]

// ✅ Correct - use .decodedCall
const calls: TxCallData[] = [
  api.tx.Assets.create({ ... }).decodedCall,
]

Using PJS Instead of PAPI

// ❌ Wrong - @polkadot/api
import { ApiPromise } from '@polkadot/api'

// ✅ Correct - polkadot-api
import { createClient } from 'polkadot-api'

Raw Strings Without Binary.fromText

// ❌ Wrong - raw string
api.tx.Assets.set_metadata({
  name: "Token",  // Type error
})

// ✅ Correct - wrapped
api.tx.Assets.set_metadata({
  name: Binary.fromText("Token"),
})

Integration with Template

If using a template with ConnectionContext:

import { useConnectionContext } from '@/hooks'

const { api, client, isConnected } = useConnectionContext()
// api is TypedApi, ready to use

Transaction management:

import { useTransaction } from '@/hooks'

const { executeTransaction } = useTransaction(toastConfig)
await executeTransaction('key', observable, params)

Validation

After implementing PAPI code:

# Check for forbidden imports
grep -r "@polkadot/api" src/
# Should return ZERO results

# Type check
tsc --noEmit

# Verify descriptors exist
ls .papi/descriptors/

Reference Links

• polkadot-api docs: https://papi.how
• Generate descriptors: Run papi or pnpm papi
• TypeScript: Let types guide you - they prevent runtime errors