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Version: 19.2.0

MichelsonMap class

One of the most attractive smart contract features is storing a substantial amount of data that the contract code can use. Although Michelson provides different structures to store data, this article's object will be its maps. Maps are hash tables that contain key/value pairs, which means that when you want to find a value in a map, you search for its key. Maps allow you to store complex data that can reference a single word or number or even more complex data like a pair!

Unlike big maps, all the values in a map are deserialized, allowing developers to access all of them at once. While maps become more expensive to use when the number of key/value pairs increases, they are well-suited for smaller databases because of Michelson's extra features (like mapping or folding) and Taquito offer on maps.

Taquito reads maps in the storage of smart contracts and translates them into an instance of the MichelsonMap class. The class and its instances expose different features that give developers much flexibility to use Michelson maps in their dapps. These features fall into four groups:

  • The instantiation: there are three different ways of creating a new MichelsonMap in Taquito
  • The general methods: they give you information about the map, for example, its size or the elements it contains
  • The key/value methods: they allow you to manipulate the keys and values in the map
  • The update methods: they transform the map itself, for example, by deleting elements or clearing out the map entirely.

This tutorial uses a simple smart contract deployed on ghostnet with a map that contains addresses as keys and tez as values. We will use all the methods available in Taquito's MichelsonMap to check the map, extract values and modify them!

Note: Taquito is written in TypeScript; we will also use TypeScript to interact with the contract storage.

Loading the smart contract storage​

This paragraph is a little reminder of how to use Taquito to fetch the storage of a smart contract:

import { TezosToolkit, MichelsonMap } from '@taquito/taquito';
import { BigNumber } from 'bignumber.js';


const contractAddress: string = 'KT1M5C76aSjpWXdoBvuzRdi3UJoC3jEzrSUW';


const Tezos = new TezosToolkit('https://ghostnet.ecadinfra.com');


const contract = await Tezos.contract.at(contractAddress);
const storage: MichelsonMap<string, BigNumber> = await contract.storage();

The setup code is pretty straightforward:
1- We import TezosToolkit and MichelsonMap from the @taquito/taquito package. We also import BigNumber from bignumber.js (Taquito installs the library) as TypeScript will need it for this particular example.
2- We instantiate the TezosToolkit object with the RPC address.
3- We fetch the contract using await Tezos.contract.at(contractAddress).
4- We extract the contract from the contract using the storage method on the ContractAbstraction object created one line above. We also type the storage variable with the MichelsonMap type, which requires 2 type arguments: the type for the key and the type for the value (the address is a string, and the tez is converted to a BigNumber by Taquito).

Creating a MichelsonMap instance​

Taquito provides three different ways of creating a new Michelson map: we can use two of them to create an empty map, and the third one is used to create a map with default values.

The most simple way is to create the instance with no argument:

const newEmptyMapWithoutArg = new MichelsonMap();

If you prefer, you can also pass an argument to the MichelsonMap constructor to indicate the type you want for the keys and the values:

// this code creates the same map as in the storage of the contract


const newEmptyMapWithArg = new MichelsonMap({
  prim: 'map',
  args: [{ prim: 'string' }, { prim: 'mutez' }],
});

Finally, you can also pass some values you want to create the instance with and let Taquito figure out the types using the fromLiteral static method:

const newMapfromLiteral = MichelsonMap.fromLiteral({
  tz1VSUr8wwNhLAzempoch5d6hLRiTh8Cjcjb: new BigNumber(123),
});

The general properties and methods: isMichelsonMap, size, has and get​

The first thing you may want to check after fetching the data from contract storage is if the part of the storage you expect to be a map is indeed a map. W can achieve this by using the isMichelsonMap static method on the MichelsonMap class:

const isMap: boolean = MichelsonMap.isMichelsonMap(storage); // true or false

Note: this is a static method, so you can use it without creating a new instance of MichelsonMap.

Once you are sure you are dealing with a map, you can check how many key/value pairs it holds with the size property:

const size: number = storage.size; // number of elements in the map

Sometimes, you don't want to do anything with the values in a map, but you want to verify whether a key appears in the map, you can then use the has method and pass it the key you are looking for:

const key: string = 'tz1MnmtP4uAcgMpeZN6JtyziXeFqqwQG6yn6';
const existsInMap: boolean = storage.has(key); // true or false

After that, you can fetch the value associated with the key you are looking for with the get method:

const key: string = 'tz1MnmtP4uAcgMpeZN6JtyziXeFqqwQG6yn6';
const valueInTez: BigNumber = storage.get(key); // value as a big number
const value: number = valueInTez.toNumber(); // returns 789000000

The key/value methods​

One of the main advantages of maps over big maps is that the key/value pairs are readily available in your dapp without any extra step. If you are looking for a simple solution to loop over all the pairs and get the key and the value, the MichelsonMap instance exposes a forEach method that allows you to get these values:

const foreachPairs: { address: string; amount: number }[] = [];
storage.forEach((val: BigNumber, key: string) => {
  foreachPairs.push({ address: key, amount: val.toNumber() / 10 ** 6 });
});
console.log(foreachPairs);

The code above will output:

[
  { address: 'tz1MnmtP4uAcgMpeZN6JtyziXeFqqwQG6yn6', amount: 789 },
  { address: 'tz1R2oNqANNy2vZhnZBJc8iMEqW79t85Fv7L', amount: 912 },
  { address: 'tz1VSUr8wwNhLAzempoch5d6hLRiTh8Cjcjb', amount: 123 },
  { address: 'tz1aSkwEot3L2kmUvcoxzjMomb9mvBNuzFK6', amount: 456 },
];

The MichelsonMap instance exposes another method that will yield the same result, albeit in a different way. The entries method is a generator function that you can use if you wish to. This is how it works:

const entriesPairs: { address: string; amount: number }[] = [];
const entries = storage.entries();
for (let entry of entries) {
  entriesPairs.push({ address: entry[0], amount: entry[1].toNumber() / 10 ** 6 });
}
console.log('entries => ' + JSON.stringify(entriesPairs) + '\n');

This code will yield the same result as the one above. A generator may be preferable according to your use case.

The same idea is available for keys and values, the keys and values methods are generators that will allow you to loop over the keys or the values of the map:

const mapKeys: string[] = [];
const keys = storage.keys();
for (let key of keys) {
  mapKeys.push(key);
}
console.log('keys => ' + mapKeys + '\n');

This example will output the following array containing all the keys of the map:

[
  'tz1MnmtP4uAcgMpeZN6JtyziXeFqqwQG6yn6',
  'tz1R2oNqANNy2vZhnZBJc8iMEqW79t85Fv7L',
  'tz1VSUr8wwNhLAzempoch5d6hLRiTh8Cjcjb',
  'tz1aSkwEot3L2kmUvcoxzjMomb9mvBNuzFK6',
];

Similarly, you can use values instead of keys to output some or all the values in the map:

const mapValues: number[] = [];
const values = storage.values();
for (let value of values) {
  mapValues.push(value.toNumber());
}
console.log('values => ' + mapValues + '\n');

This command will output all the values of the map inside an array:

[789000000, 912000000, 123000000, 456000000];

The update methods​

Although reading and organizing the keys or the values fetched from a Michelson map is an everyday use case, you may also want to modify a map, for example, before originating a new contract. Taquito also thought about it and provided different methods to add or remove key/value pairs from a map.

First, you can use the set method to add a new value to an instance of MichelsonMap:

console.log(`previous size => ${storage.size} elements`); // 4 elements


storage.set('tz1TfRXkAxbQ2BFqKV2dF4kE17yZ5BmJqSAP', new BigNumber(345));


console.log(`new size => ${storage.size} elements \n`); // 5 elements

This command adds a new entry in the map with the first argument's address and the BigNumber being the value.

Note: it is essential to use new BigNumber(345) for the value and not merely 345 as TypeScript will throw a type error because earlier, we set the type argument of the MichelsonMap to BigNumber.

You can also delete one of the entries of the map with the delete method:

console.log(`delete: previous size => ${storage.size} elements`); // 5 elements


storage.delete('tz1MnmtP4uAcgMpeZN6JtyziXeFqqwQG6yn6');


console.log(`delete: new size => ${storage.size} elements \n`); // 4 elements

Note: deleting a key that doesn't exist doesn't throw an error; it will just not affect the map.

To finish, you can also delete all the entries in a Michelson map if you want with the clear method:

storage.clear();
console.log(`clear: new size => ${storage.size} element`); // 0 element

To go further​

If you want to know more about MichelsonMap and some advanced usages (for example, how to use pairs as the map keys), you can learn in the advanced tutorial available in the Taquito documentation.

April 2021, Taquito version 8.1.0

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