Skip to main content
Version: 6.11.0

Data transformation

This guide is the most important of all this documentation. Take your time, and read it carefully...

Cairo contracts and JavaScript/TypeScript languages do not have the same types of data. To exchange data with Starknet, the data have to be transformed and formatted in a list of numbers.

So, it's necessary to prepare the data before sending them to a contract. On the other side, when a contract sends data to your DAPP (the result of a call), you also have to transform them before using them in your code.

In Starknet.js, you can perform these transformations manually, but you can take advantage of methods that perform these transformations.

Types of data


Cairo has 2 versions, involving 2 types of data:

  • Cairo 0: here, everything is felt, an integer on 251 bits.
    Available: array, struct, tuple, named tuple, or a mix of these elements.
  • Cairo 1: with plethora of literal types: u8, u16, u32, usize, u64, u128, felt252, u256, bool, address, eth address, classHash.
    Available: array, struct, tuple, bytes31, byteArray, enums or a mix of these elements.

Starknet.js is compatible with both versions.


Starknet is waiting for a list of felts, and answers with the same format.

JavaScript / TypeScript

These types do not exist in JS/TS - you have Number, bigInt, string, array, objects... and types defined in libraries.

In Starknet.js, it's a bit ... complicated: you have the BigNumberish type and it can include:

  • String (representing a number): "123", "0xabc2"
  • Number (max 53 bits): 123
  • BigInt (max 255 bits): 12345612345n
import { BigNumberish } from 'starknet';
const decimals: BigNumberish = 18;

Preparation of data before delivery

If your Cairo smart contract is waiting for a:

felt, u8, u16, u32, usize, u64, u128, felt252, ContractAddress, EthAddress, ClassHash

Starknet is waiting for a felt.
You can send to Starknet.js methods: bigNumberish.

await myContract.my_function(12, '13', '0xe', 15n);

EthAddress is limited to 160 bits. felt, felt252, ClassHash and ContractAddress are limited to 252 bits.


Starknet is waiting for a felt, containing 0 or 1.
You can send to Starknet.js methods: boolean, bigNumberish.

await myContract.my_function(true, 1);


Starknet is waiting for 2 felts, the first including the lowest 128 bits, the second including the 128 highest bits.
You can send to Starknet.js methods: bigNumberish (Cairo 1 only), Uint256 object (both Cairo 0 & 1).

await myContract0.my_function({ low: 100, high: 0 }); // Cairo 0 & 1 contract
await myContract1.my_function(cairo.uint256(100)); // Cairo 0 & 1 contract
await myContract2.my_function(12345678, '13456789765', '0xe23a40b543f', 1534566734334n); // Cairo 1 contract

In specific cases that we will see hereunder, you can use an object, with the following format:

const a1: Uint256 = cairo.uint256(
const a2: Uint256 = {
low: '0xeb5337d9a885be319366b5205a414fdd',
high: '0x05f7cd1fd465baff2ba9d2d1501ad0a2',
const a3: Uint256 = { low: a1.low, high: a1.high };


Starknet is waiting for 4 u128, the first one has the lowest weight.
You can send to Starknet.js methods: bigNumberish or Uint512 object.

await myContract0.my_function(553844998243714947043252949842317834769n);
await myContract1.my_function(
await myContract2.my_function(12345678, '13456789765', '0xe23a40b543f', 1534566734334n);

In specific cases, you can use an object, with the following format:

const a2: Uint512 = {
limb0: '0xeb5337d9a885be310x9365205a414fdd',
limb1: '0x1fd465baff2ba9d2d1501ad0a2eb5337',
limb2: '0x05f7cd1fd465baff2ba9d2d1501ad0a2',
limb3: '0x2eb5337d9a885be319366b5205a414fd',

shortString or bytes31

For a shortString, Starknet is waiting for a felt, including 31 ASCII characters max.
You can send to Starknet.js methods: string, bigNumberish.

bytes31 is similar to shortString.
You can send to Starknet.js methods: string.

await myContract.my_function('Token', '0x0x534e5f4d41494e'); // send 2 shortStrings

To encode yourself a string:

const encStr: string = shortString.encodeShortString('Stark');

To decode yourself a string:

const decStr: string = shortString.decodeShortString('0x7572692f706963742f7433382e6a7067');

The result is: "uri/pict/t38.jpg"

longString or ByteArray

longString is a string that may contain more than 31 characters.
Starknet is waiting for an array of felt: string_len, string1, string2, ...
You can send to Starknet.js methods: string, bigNumberish[].

ByteArray is similar to longString.
Starknet is waiting for a specific struct.
You can send to Starknet.js methods: string.

await myContract.my_function('http://addressOfMyERC721pictures/image1.jpg');

To force to send a shortString as a ByteArray with CallData.compile():

const myCalldata = Calldata.compile([byteArray.byteArrayFromString('Take care.')]);

If you want to split yourself your longString in 31 chars substrings:

const splitted: string[] = shortString.splitLongString(

If you want to split your longString in an array of felts:

const longString: string[] = shortString
.map((str) => shortString.encodeShortString(str));


Starknet is waiting for a list of felts.
You can send it to Starknet.js methods: cairo.tuple(), object.

const myTpl = cairo.tuple('0x0a', 200);
await myContract.my_function(myTpl);

To construct your tuple:

const myTpl = { '0': '0x0a', '1': 200 };

Named tuple

Only for Cairo 0

Starknet is waiting for a list of felts.
You can send to Starknet.js methods: an object, cairo.tuple(), list of bigNumberish.
From this ABI:

"name": "data2",
"type": "(min: felt, max: felt)"

You can create this code:

const namedTup = { min: '0x4e65ac6', max: 296735486n };
await myContract.my_function(namedTup);

It's not mandatory to create manually an object conform to the Cairo 0 named tuple ; you can just use the cairo.tuple() function.

Ethereum public key

If your abi is requesting this type : core::starknet::secp256k1::Secp256k1Point, it means that you have probably to send an Ethereum full public key. Example :

"type": "constructor",
"name": "constructor",
"inputs": [
"name": "public_key",
"type": "core::starknet::secp256k1::Secp256k1Point"
  • If you are using a calldata construction method using the Abi, you have just to use a 512 bits number (so, without parity) :
const privateKeyETH = '0x45397ee6ca34cb49060f1c303c6cb7ee2d6123e617601ef3e31ccf7bf5bef1f9';
const ethSigner = new EthSigner(privateKeyETH);
const ethFullPublicKey = await ethSigner.getPubKey(); // 512 bits number
const myCallData = new CallData(ethAccountAbi);
const accountETHconstructorCalldata = myCallData.compile('constructor', {
public_key: ethFullPublicKey,
  • If you are using a calldata construction method without the Abi, you have to send a tuple of 2 u256 :
const ethFullPublicKey =
'0x0178bb97615b49070eefad71cb2f159392274404e41db748d9397147cb25cf597ebfcf2f399e635b72b99b8f76e9080763c65a42c842869815039d912150ddfe'; // 512 bits number
const pubKeyETH = encode.addHexPrefix(encode.removeHexPrefix(ethFullPublicKey).padStart(128, '0'));
const pubKeyETHx = cairo.uint256(addAddressPadding(encode.addHexPrefix(pubKeyETH.slice(2, -64))));
const pubKeyETHy = cairo.uint256(addAddressPadding(encode.addHexPrefix(pubKeyETH.slice(-64))));
const accountETHconstructorCalldata = CallData.compile([cairo.tuple(pubKeyETHx, pubKeyETHy)]);


Starknet is waiting for a list of felts.
You can send to Starknet.js methods: an object.

const myStruct = { type: 'TR1POST', tries: 8, isBridged: true };
await myContract.my_function(myStruct);

array, span

Starknet is waiting for an array of felts: array_len, array1, array2, ...
You can send it to Starknet.js methods: bigNumberish[].

Const myArray = [10, "0xaa", 567n];
await myContract.my_function(myArray);

Do not add the array_len parameter before your array. Starknet.js will manage this element automatically.

It's also applicable for Cairo Span type.

Complex types

You can mix and nest literals, arrays, structs, and tuples.

Starknet is waiting for a list of felts.
All these examples are valid:

type Order2 = {
p1: BigNumberish;
p2: BigNumberish[];
}; // struct
const myOrder2: Order2 = {
p1: 17,
p2: [234, 467456745457n, '0x56ec'],
const param1 = cairo.tuple(cairo.tuple(34, '0x5e'), 234n);
const param2 = [
[200, 201],
[202, 203],
[204, 205],
const param3 = [myOrder2, myOrder2];
const param4 = [cairo.tuple(251, 40000n), cairo.tuple(252, 40001n)];
await myContract.my_function(param1, param2, param3, param4);

Authorized types for Starknet.js methods

There are 12 methods using contract parameters. Some types are authorized for each method:

List of parameters

Only meta-class methods are using a list of parameters (as illustrated in the previous chapter).
A Meta-Class is a Class that has any of its properties determined at run-time. The Contract object uses a Contract's ABI to determine what methods are available.

await myContract.my_function('TOKEN', '13', [10, 11, 12], 135438734812n);
// or
const functionName = 'my_function';
await myContract[functionName]('TOKEN', '13', [10, 11, 12], 135438734812n);

Array of parameters

An array of parameters can be used as input:

const myParams = [{ x: 100, y: 200 }, 13, [10, 11, 12], cairo.uint256('0x295fa652e32b')];
const txResp = await account0.execute({
contractAddress: testAddress,
entrypoint: 'change_activity',
calldata: myParams,

All Starknet.js methods accept this type of input, except meta-class, which needs 3 dots prefix:

const myParams = ['TOKEN', '13', [10, 11, 12], 135438734812n];
await myContract.my_function(...myParams);
// or
const functionName = 'my_function';
await myContract[functionName](...myParams);

Objects properties have to be ordered in accordance with the ABI.

Object (without ABI conformity check)

The use of objects allows a clear representation of the list of parameters:

const myParams = {
name: 'TOKEN',
decimals: '13',
amount: 135438734812n,
const deployResponse = await myAccount.deployContract({
classHash: contractClassHash,
constructorCalldata: myParams,

This type is available for: CallData.compile(), hash.calculateContractAddressFromHash, account.deployContract, account.deployAccount, account.execute


Objects properties have to be ordered in accordance with the ABI.

Object (with ABI conformity check)

This is the recommended type of input to use, especially for complex ABI.

const myFalseUint256 = { high: 1, low: 23456 }; // wrong order; should be low first
type Order2 = {
p1: BigNumberish;
p2: BigNumberish[];
const myOrder2bis: Order2 = {
// wrong order; p1 should be first
p2: [234, 467456745457n, '0x56ec'],
p1: '17',
const functionParameters: RawArgsObject = {
//wrong order; all properties are mixed
active: true,
symbol: 'NIT',
initial_supply: myFalseUint256,
recipient: account0.address,
decimals: 18,
tupOfTup: cairo.tuple(cairo.tuple(34, '0x5e'), myFalseUint256),
card: myOrder2bis,
longText: 'Zorg is back, for ever, here and everywhere',
array1: [100, 101, 102],
array2: [
[200, 201],
[202, 203],
[204, 205],
array3: [myOrder2bis, myOrder2bis],
array4: [myFalseUint256, myFalseUint256],
tuple1: cairo.tuple(40000n, myOrder2bis, [54, 55n, '0xae'], 'texte'),
name: 'niceToken',
array5: [cairo.tuple(251, 40000n), cairo.tuple(252, 40001n)],
const contractCallData: CallData = new CallData(compiledContractSierra.abi);
const myCalldata: Calldata = contractCallData.compile('constructor', functionParameters);
const deployResponse = await account0.deployContract({
classHash: contractClassHash,
constructorCalldata: myCalldata,
// or
const myCall: Call = myContract.populate('setup_elements', functionParameters);
const tx = await account0.execute(myCall);
// or
const myCall: Call = myContract.populate('get_elements', functionParameters);
const res = await myContract.get_elements(myCall.calldata);

It can be used only with methods that know the abi: Contract.populate, myCallData.compile.
Starknet.js will perform a full check of conformity with the ABI of the contract, reorder the object's properties if necessary, stop if something is wrong or missing, remove not requested properties, and convert everything to Starknet format.
Starknet.js will alert you earlier of errors in your parameters (with human comprehensible words), before the call to Starknet. So, no more incomprehensible Starknet messages due to parameters construction.

If a property array_len has been added before an array, this property is ignored as it's automatically managed by Starknet.js.

Call, or Call[]

A Call is an object with this format:

type Call = {
contractAddress: string;
entrypoint: string;
calldata?: RawArgs;

...and is only authorized with Account.execute . It can be generated manually or by Contract.populate():

const myCall: Call = myContract.populate('get_component', [100, recipient]);
// or
const myCall: Call = {
contractAddress: tokenContract.address,
entrypoint: 'get_component',
calldata: CallData.compile([100, recipient]),

const tx = await account0.execute(myCall);

It's particularly interesting when you want to invoke a function several times in the same transaction:

const myCall1: Call = myContract.populate('mint', { type: 7, qty: 10 });
const myCall2: Call = myContract.populate('mint', { type: 21, qty: 3 });
const myCall3: Call = myContract.populate('mint', { type: 2, qty: 1 });
const tx = await account0.execute([myCall1, myCall2, myCall3]);

Array of strings (representing numbers)

This type is particularly useful when you need the maximum performance and speed in your code; You have no automatic transformation, no checks with ABI, and no parsing.

You provide to starknet.js the low-level data expected by Starknet:

const specialParameters: Calldata = [
const getResponse = await myAccount.get_bal(specialParameters, { parseRequest: false });

To use with parseRequest: false (see hereunder).

Summary table for arguments

These types of arguments can't be used at your convenience everywhere. Here is a table showing which types can be used in which function:

Functionarray of parametersordered objectnon ordered objectCall & MultiCalllist of parametersarray of strings (*)array of strings (**)
TypeScript typeN/A{} RawArgsArray{} RawArgsObjectCall & Call[]...[]string[]string[]
contract.metaClass() contract[metaclass]()✔️✔️✔️ / contract.invoke✔️✔️✔️

(with 3 params, incl. calldata)



account.deployContract / Account✔️✔️✔️
hash. calculateContract AddressFromHash✔️✔️✔️

(*) = with parseRequest: false
(**) = result of Calldata.compile, myCallData.compile, myContract.populate().calldata

Receive data from a Cairo contract

When you perform a call, the result depends on the contract language:

  • In Cairo 0, the answer is an object, with keys using the Cairo variable's names.


const amount = res.amount;
  • In Cairo 1, the result is a variable:
const amount =;
Type in Cairo 1Cairo 1 codeType expected in JS/TSJS/TS function to recover data
u8, u16, u32, usize, u64, u128, felt252, addressfunc get_v()->u128bigintconst res: bigint =
string representing an hex numberconst
const address: string = num.toHex(res);
u8, u16, u32, usizefunc get_v() -> u16number (53 bits max)const
const total: number = Number(res)
u256 (255 bits max)func get_v() -> u256bigintconst res: bigint =
u512 (512 bits max)func get_v() -> u512bigintconst res: bigint =
array of u8, u16, u32, usize, u64, u128, felt252, addressfunc get_v() -> Array<u64>bigint[]const res: bigint[] =
bytes31 (31 ASCII characters max)func get_v() -> bytes31stringconst res: string =
felt252 (31 ASCII characters max)func get_v() -> felt252stringconst res =
const title:string = shortString.decodeShortstring(res);
longStringfunc get_v() -> Array<felt252>stringconst
const longString = (shortStr: bigint) => { return shortString.decodeShortString( num.toHex( shortStr)) }).join("");
ByteArrayfunc get_v() -> ByteArraystringconst res: string =
Tuplefunc get_v() -> (felt252, u8)Object {"0": bigint, "1": bigint}const res =
const res0: bigint = res["0"];
const results: bigint[] = Object.values(res)
Struct func get_v() -> MyStructMyStruct = { account: bigint, amount: bigint}const res: MyStruct =
complex arrayfunc get_v() -> Array<fMyStruct>MyStruct[]const res: MyStruct[] =

If you don't know if your Contract object is interacting with a Cairo 0 or a Cairo 1 contract, you have these methods:

import { cairo } from 'starknet';
const isCairo1: boolean = myContract.isCairo1();
const isAbiCairo1: boolean = cairo.isCairo1Abi(myAbi);

Parse configuration


If for any reason (mainly for speed of processing), you want to define yourself the low-level parameters to send to Starknet, you can use the parseRequest option.
Parameters are an array of strings (representing numbers).

const txH = await myContract.send_tk(['2036735872918048433518', '5130580', '18'], {
parseRequest: false,


If for any reason, you want to receive a low-level answer from Starknet, you can use the parseResponse option.

const result = await'get_bals', 100n, { parseResponse: false });

The answer is an array of strings (representing numbers).


As seen above, the strings returned by Starknet are not automatically parsed, because ABI does not inform when a contract returns a string.
But there is a way to have automatic parsing of a string.

For example, if a contract returns a struct containing a shortString and a longString:

{ name: felt252, description: Array<felt252> }

You can automate the string parsing with:

const formatAnswer = { name: 'string', description: 'string' };
const result = await myContract.get_text(calldata, {
parseRequest: true,
parseResponse: true,
formatResponse: formatAnswer,

The result will be an object, with 2 strings:

{ name: "Organic", description: "The best way to read a long string!!!" }

Tool to learn how to encode/decode

A DAPP has been created to learn how to encode/decode with Starknet.js : Startnet-encode-decode.
It's also a convenient tool for the exploration of any contract ABI.

Follow these links :
Tuto :