SDK Reference

Overview

This reference documents methods available in the SDK that can be accessed via FCL, and explains in detail how these methods work. FCL/SDKs are open source, and you can use them according to the licence.

The library client specifications can be found here:

Getting Started

Installing

NPM:


_10npm install --save @onflow/fcl @onflow/types

Yarn:


_10yarn add @onflow/fcl @onflow/types

Importing the Library


_10import * as fcl from "@onflow/fcl"
_10import * as types from "@onflow/types"

Connect

By default, the library uses HTTP to communicate with the access nodes and it must be configured with the correct access node API URL. An error will be returned if the host is unreachable.

📖The HTTP/REST API information can be found here. The public Flow HTTP/REST access nodes are accessible at:

Testnet https://rest-testnet.onflow.org
Mainnet https://rest-mainnet.onflow.org
Local Emulator 127.0.0.1:8888

Example:


_10import { config } from "@onflow/fcl"
_10
_10config({
_10  "accessNode.api": "https://rest-testnet.onflow.org"
_10})

Querying the Flow Network

After you have established a connection with an access node, you can query the Flow network to retrieve data about blocks, accounts, events and transactions. We will explore how to retrieve each of these entities in the sections below.

Get Blocks

Query the network for block by id, height or get the latest block.

📖 Block ID is SHA3-256 hash of the entire block payload. This hash is stored as an ID field on any block response object (ie. response from GetLatestBlock).

📖 Block height expresses the height of the block on the chain. The latest block height increases by one for every valid block produced.

Examples

This example depicts ways to get the latest block as well as any other block by height or ID:


_10import * as fcl from "@onflow/fcl";
_10
_10// Get latest block
_10const latestBlock = await fcl.latestBlock(true); // If true, get the latest sealed block
_10
_10// Get block by ID (uses builder function)
_10await fcl.send([fcl.getBlock(), fcl.atBlockId("23232323232")]).then(fcl.decode);
_10
_10// Get block at height (uses builder function)
_10await fcl.send([fcl.getBlock(), fcl.atBlockHeight(123)]).then(fcl.decode)

Result output: BlockObject

Get Account

Retrieve any account from Flow network's latest block or from a specified block height.

📖 Account address is a unique account identifier. Be mindful about the 0x prefix, you should use the prefix as a default representation but be careful and safely handle user inputs without the prefix.

An account includes the following data:

Address: the account address.
Balance: balance of the account.
Contracts: list of contracts deployed to the account.
Keys: list of keys associated with the account.

Examples

Example depicts ways to get an account at the latest block and at a specific block height:


_10import * as fcl from "@onflow/fcl";
_10
_10// Get account from latest block height
_10const account = await fcl.account("0x1d007d755706c469");
_10
_10// Get account at a specific block height
_10fcl.send([
_10  fcl.getAccount("0x1d007d755706c469"),
_10  fcl.atBlockHeight(123)
_10]);

Result output: AccountObject

Get Transactions

Retrieve transactions from the network by providing a transaction ID. After a transaction has been submitted, you can also get the transaction result to check the status.

📖 Transaction ID is a hash of the encoded transaction payload and can be calculated before submitting the transaction to the network.

⚠️ The transaction ID provided must be from the current spork.

📖 Transaction status represents the state of a transaction in the blockchain. Status can change until it is finalized.

Status	Final	Description
UNKNOWN	❌	The transaction has not yet been seen by the network
PENDING	❌	The transaction has not yet been included in a block
FINALIZED	❌	The transaction has been included in a block
EXECUTED	❌	The transaction has been executed but the result has not yet been sealed
SEALED	✅	The transaction has been executed and the result is sealed in a block
EXPIRED	✅	The transaction reference block is outdated before being executed


_16import * as fcl from "@onflow/fcl";
_16
_16// Snapshot the transaction at a point in time
_16fcl.tx(transactionId).snapshot();
_16
_16// Subscribe to a transaction's updates
_16fcl.tx(transactionId).subscribe(callback);
_16
_16// Provides the transaction once the status is finalized
_16fcl.tx(transactionId).onceFinalized();
_16
_16// Provides the transaction once the status is executed
_16fcl.tx(transactionId).onceExecuted();
_16
_16// Provides the transaction once the status is sealed
_16fcl.tx(transactionId).onceSealed();

Result output: TransactionStatusObject

Get Events

Retrieve events by a given type in a specified block height range or through a list of block IDs.

📖 Event type is a string that follow a standard format:


_10A.{contract address}.{contract name}.{event name}

Please read more about events in the documentation. The exception to this standard are core events, and you should read more about them in this document.

📖 Block height range expresses the height of the start and end block in the chain.

Examples

Example depicts ways to get events within block range or by block IDs:


_22import * as fcl from "@onflow/fcl";
_22
_22// Get events at block height range
_22await fcl
_22  .send([
_22    fcl.getEventsAtBlockHeightRange(
_22      "A.7e60df042a9c0868.FlowToken.TokensWithdrawn", // event name
_22      35580624, // block to start looking for events at
_22      35580624 // block to stop looking for events at
_22    ),
_22  ])
_22  .then(fcl.decode);
_22
_22// Get events from list of block ids
_22await fcl
_22  .send([
_22    fcl.getEventsAtBlockIds("A.7e60df042a9c0868.FlowToken.TokensWithdrawn", [
_22      "c4f239d49e96d1e5fbcf1f31027a6e582e8c03fcd9954177b7723fdb03d938c7",
_22      "5dbaa85922eb194a3dc463c946cc01c866f2ff2b88f3e59e21c0d8d00113273f",
_22    ]),
_22  ])
_22  .then(fcl.decode);

Result output: EventObject

Get Collections

Retrieve a batch of transactions that have been included in the same block, known as collections. Collections are used to improve consensus throughput by increasing the number of transactions per block and they act as a link between a block and a transaction.

📖 Collection ID is SHA3-256 hash of the collection payload.

Example retrieving a collection:


_10import * as fcl from "@onflow/fcl";
_10
_10const collection = await fcl
_10  .send([
_10    fcl.getCollection(
_10      "cccdb0c67d015dc7f6444e8f62a3244ed650215ed66b90603006c70c5ef1f6e5"
_10    ),
_10  ])
_10  .then(fcl.decode);

Result output: CollectionObject

Execute Scripts

Scripts allow you to write arbitrary non-mutating Cadence code on the Flow blockchain and return data. You can learn more about Cadence here and scripts here, but we are now only interested in executing the script code and getting back the data.

We can execute a script using the latest state of the Flow blockchain or we can choose to execute the script at a specific time in history defined by a block height or block ID.

📖 Block ID is SHA3-256 hash of the entire block payload, but you can get that value from the block response properties.

📖 Block height expresses the height of the block in the chain.


_15import * as fcl from "@onflow/fcl";
_15
_15const result = await fcl.query({
_15  cadence: `
_15    access(all) fun main(a: Int, b: Int, addr: Address): Int {
_15      log(addr)
_15      return a + b
_15    }
_15  `,
_15  args: (arg, t) => [
_15    arg(7, t.Int), // a: Int
_15    arg(6, t.Int), // b: Int
_15    arg("0xba1132bc08f82fe2", t.Address), // addr: Address
_15  ],
_15});

Example output:


_10console.log(result); // 13

Mutate Flow Network

Flow, like most blockchains, allows anybody to submit a transaction that mutates the shared global chain state. A transaction is an object that holds a payload, which describes the state mutation, and one or more authorizations that permit the transaction to mutate the state owned by specific accounts.

Transaction data is composed and signed with help of the SDK. The signed payload of transaction then gets submitted to the access node API. If a transaction is invalid or the correct number of authorizing signatures are not provided, it gets rejected.

Transactions

A transaction is nothing more than a signed set of data that includes script code which are instructions on how to mutate the network state and properties that define and limit it's execution. All these properties are explained bellow.

📖 Script field is the portion of the transaction that describes the state mutation logic. On Flow, transaction logic is written in Cadence. Here is an example transaction script:


_10transaction(greeting: String) {
_10  execute {
_10    log(greeting.concat(", World!"))
_10  }
_10}

📖 Arguments. A transaction can accept zero or more arguments that are passed into the Cadence script. The arguments on the transaction must match the number and order declared in the Cadence script. Sample script from above accepts a single String argument.

📖 Proposal key must be provided to act as a sequence number and prevent replay and other potential attacks.

Each account key maintains a separate transaction sequence counter; the key that lends its sequence number to a transaction is called the proposal key.

A proposal key contains three fields:

Account address
Key index
Sequence number

A transaction is only valid if its declared sequence number matches the current on-chain sequence number for that key. The sequence number increments by one after the transaction is executed.

📖 Payer is the account that pays the fees for the transaction. A transaction must specify exactly one payer. The payer is only responsible for paying the network and gas fees; the transaction is not authorized to access resources or code stored in the payer account.

📖 Authorizers are accounts that authorize a transaction to read and mutate their resources. A transaction can specify zero or more authorizers, depending on how many accounts the transaction needs to access.

The number of authorizers on the transaction must match the number of &Account parameters declared in the prepare statement of the Cadence script.

Example transaction with multiple authorizers:


_10transaction {
_10  prepare(authorizer1: &Account, authorizer2: &Account) { }
_10}

📖 Gas limit is the limit on the amount of computation a transaction requires, and it will abort if it exceeds its gas limit. Cadence uses metering to measure the number of operations per transaction. You can read more about it in the Cadence documentation.

The gas limit depends on the complexity of the transaction script. Until dedicated gas estimation tooling exists, it's best to use the emulator to test complex transactions and determine a safe limit.

📖 Reference block specifies an expiration window (measured in blocks) during which a transaction is considered valid by the network. A transaction will be rejected if it is submitted past its expiry block. Flow calculates transaction expiry using the reference block field on a transaction. A transaction expires after 600 blocks are committed on top of the reference block, which takes about 10 minutes at average Mainnet block rates.

Mutate

FCL "mutate" does the work of building, signing, and sending a transaction behind the scenes. In order to mutate the blockchain state using FCL, you need to do the following:


_16import * as fcl from "@onflow/fcl"
_16
_16await fcl.mutate({
_16  cadence: `
_16    transaction(a: Int) {
_16      prepare(acct: &Account) {
_16        log(acct)
_16        log(a)
_16      }
_16    }
_16  `,
_16  args: (arg, t) => [
_16    arg(6, t.Int)
_16  ],
_16  limit: 50
_16})

Flow supports great flexibility when it comes to transaction signing, we can define multiple authorizers (multi-sig transactions) and have different payer account than proposer. We will explore advanced signing scenarios bellow.

Single party, single signature

Proposer, payer and authorizer are the same account (0x01).
Only the envelope must be signed.
Proposal key must have full signing weight.

Account	Key ID	Weight
`0x01`	1	1000


_56// There are multiple ways to acheive this
_56import * as fcl from "@onflow/fcl"
_56
_56// FCL provides currentUser as an authorization function
_56await fcl.mutate({
_56  cadence: `
_56    transaction {
_56      prepare(acct: &Account) {}
_56    }
_56  `,
_56  proposer: currentUser,
_56  payer: currentUser,
_56  authorizations: [currentUser],
_56  limit: 50,
_56})
_56
_56// Or, simplified
_56
_56mutate({
_56  cadence: `
_56    transaction {
_56      prepare(acct: &Account) {}
_56    }
_56  `,
_56  authz: currentUser, // Optional. Will default to currentUser if not provided.
_56  limit: 50,
_56})
_56
_56
_56// Or, create a custom authorization function
_56const authzFn = async (txAccount) => {
_56  return {
_56    ...txAccount,
_56    addr: "0x01",
_56    keyId: 0,
_56    signingFunction: async(signable) => {
_56      return {
_56        addr: "0x01",
_56        keyId: 0,
_56        signature
_56      }
_56    }
_56  }
_56}
_56
_56mutate({
_56  cadence: `
_56    transaction {
_56      prepare(acct: &Account) {}
_56    }
_56  `,
_56  proposer: authzFn,
_56  payer: authzFn,
_56  authorizations: [authzFn],
_56  limit: 50,
_56})

Single party, multiple signatures

Proposer, payer and authorizer are the same account (0x01).
Only the envelope must be signed.
Each key has weight 500, so two signatures are required.

Account	Key ID	Weight
`0x01`	1	500
`0x01`	2	500


_42import * as fcl from "@onflow/fcl"
_42
_42const authzFn = async (txAccount) => {
_42  return [
_42    {
_42      ...txAccount,
_42      addr: "0x01",
_42      keyId: 0,
_42      signingFunction: async(signable) => {
_42        return {
_42          addr: "0x01",
_42          keyId: 0,
_42          signature
_42        }
_42      }
_42    },
_42    {
_42      ...txAccount,
_42      addr: "0x01",
_42      keyId: 1,
_42      signingFunction: async(signable) => {
_42        return {
_42          addr: "0x01",
_42          keyId: 1,
_42          signature
_42        }
_42      }
_42    }
_42  ]
_42}
_42
_42mutate({
_42  cadence: `
_42    transaction {
_42      prepare(acct: &Account) {}
_42    }
_42  `,
_42  proposer: authzFn,
_42  payer: authzFn,
_42  authorizations: [authzFn],
_42  limit: 50,
_42})

Multiple parties

Proposer and authorizer are the same account (0x01).
Payer is a separate account (0x02).
Account 0x01 signs the payload.
Account 0x02 signs the envelope.
- Account 0x02 must sign last since it is the payer.

Account	Key ID	Weight
`0x01`	1	1000
`0x02`	3	1000


_43import * as fcl from "@onflow/fcl"
_43
_43const authzFn = async (txAccount) => {
_43  return {
_43    ...txAccount,
_43    addr: "0x01",
_43    keyId: 0,
_43    signingFunction: async(signable) => {
_43      return {
_43        addr: "0x01",
_43        keyId: 0,
_43        signature
_43      }
_43    }
_43  }
_43}
_43
_43const authzTwoFn = async (txAccount) => {
_43  return {
_43    ...txAccount,
_43    addr: "0x02",
_43    keyId: 0,
_43    signingFunction: async(signable) => {
_43      return {
_43        addr: "0x02",
_43        keyId: 0,
_43        signature
_43      }
_43    }
_43  }
_43}
_43
_43mutate({
_43  cadence: `
_43    transaction {
_43      prepare(acct: &Account) {}
_43    }
_43  `,
_43  proposer: authzFn,
_43  payer: authzTwoFn,
_43  authorizations: [authzFn],
_43  limit: 50,
_43})

Multiple parties, two authorizers

Proposer and authorizer are the same account (0x01).
Payer is a separate account (0x02).
Account 0x01 signs the payload.
Account 0x02 signs the envelope.
- Account 0x02 must sign last since it is the payer.
Account 0x02 is also an authorizer to show how to include two &Account objects into an transaction

Account	Key ID	Weight
`0x01`	1	1000
`0x02`	3	1000


_43import * as fcl from "@onflow/fcl"
_43
_43const authzFn = async (txAccount) => {
_43  return {
_43    ...txAccount,
_43    addr: "0x01",
_43    keyId: 0,
_43    signingFunction: async(signable) => {
_43      return {
_43        addr: "0x01",
_43        keyId: 0,
_43        signature
_43      }
_43    }
_43  }
_43}
_43
_43const authzTwoFn = async (txAccount) => {
_43  return {
_43    ...txAccount,
_43    addr: "0x02",
_43    keyId: 0,
_43    signingFunction: async(signable) => {
_43      return {
_43        addr: "0x02",
_43        keyId: 0,
_43        signature
_43      }
_43    }
_43  }
_43}
_43
_43mutate({
_43  cadence: `
_43    transaction {
_43      prepare(acct: &Account, acct2: &Account) {}
_43    }
_43  `,
_43  proposer: authzFn,
_43  payer: authzTwoFn,
_43  authorizations: [authzFn, authzTwoFn],
_43  limit: 50,
_43})

Multiple parties, multiple signatures

Proposer and authorizer are the same account (0x01).
Payer is a separate account (0x02).
Account 0x01 signs the payload.
Account 0x02 signs the envelope.
- Account 0x02 must sign last since it is the payer.
Both accounts must sign twice (once with each of their keys).

Account	Key ID	Weight
`0x01`	1	500
`0x01`	2	500
`0x02`	3	500
`0x02`	4	500


_71import * as fcl from "@onflow/fcl"
_71
_71const authzFn = async (txAccount) => {
_71  return [
_71    {
_71      ...txAccount,
_71      addr: "0x01",
_71      keyId: 0,
_71      signingFunction: async(signable) => {
_71        return {
_71          addr: "0x01",
_71          keyId: 0,
_71          signature
_71        }
_71      }
_71    },
_71    {
_71      ...txAccount,
_71      addr: "0x01",
_71      keyId: 1,
_71      signingFunction: async(signable) => {
_71        return {
_71          addr: "0x01",
_71          keyId: 1,
_71          signature
_71        }
_71      }
_71    }
_71  ]
_71}
_71
_71const authzTwoFn = async (txAccount) => {
_71  return [
_71    {
_71      ...txAccount,
_71      addr: "0x02",
_71      keyId: 0,
_71      signingFunction: async(signable) => {
_71        return {
_71          addr: "0x02",
_71          keyId: 0,
_71          signature
_71        }
_71      }
_71    },
_71    {
_71      ...txAccount,
_71      addr: "0x02",
_71      keyId: 1,
_71      signingFunction: async(signable) => {
_71        return {
_71          addr: "0x02",
_71          keyId: 1,
_71          signature
_71        }
_71      }
_71    }
_71  ]
_71}
_71
_71mutate({
_71  cadence: `
_71    transaction {
_71      prepare(acct: &Account) {}
_71    }
_71  `,
_71  proposer: authzFn,
_71  payer: authzTwoFn,
_71  authorizations: [authzFn],
_71  limit: 50,
_71})

After a transaction has been built and signed, it can be sent to the Flow blockchain where it will be executed. If sending was successful you can then retrieve the transaction result.

Overview​

Getting Started​

Installing​

Importing the Library​

Connect​

Querying the Flow Network​

Get Blocks​

Examples​

Get Account​

Examples​

Get Transactions​

Get Events​

Examples​

Get Collections​

Execute Scripts​

Mutate Flow Network​

Transactions​

Mutate​

Single party, single signature​

Single party, multiple signatures​

Multiple parties​

Multiple parties, two authorizers​

Multiple parties, multiple signatures​

Overview

Getting Started

Installing

Importing the Library

Connect

Querying the Flow Network

Get Blocks

Examples

Get Account

Examples

Get Transactions

Get Events

Examples

Get Collections

Execute Scripts

Mutate Flow Network

Transactions

Mutate

Single party, single signature

Single party, multiple signatures

Multiple parties

Multiple parties, two authorizers

Multiple parties, multiple signatures