Token metadata and URI in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity function fetching token URI?

Consider this Solidity function that returns a token URI based on token ID. What will be the output when calling tokenURI(42)?

Blockchain / Solidity

function tokenURI(uint256 tokenId) public view returns (string memory) {
    string memory baseURI = "https://example.com/metadata/";
    return string(abi.encodePacked(baseURI, Strings.toString(tokenId), ".json"));
}

A "https://example.com/metadata/42.json"

B "https://example.com/metadata/token42.json"

C "https://example.com/metadata/42"

D "https://example.com/metadata/.json"

💡 Hint

Look at how the base URI and token ID are concatenated with the file extension.

The function concatenates the base URI, the string form of the token ID, and the '.json' extension. For tokenId 42, it returns 'https://example.com/metadata/42.json'.

🧠 Conceptual

intermediate

Which standard defines the metadata URI for ERC-721 tokens?

In Ethereum, which standard specifies how token metadata URI should be exposed for NFTs?

A ERC-721

B ERC-20

C ERC-1155

D ERC-777

💡 Hint

This standard is the original NFT standard.

ERC-721 is the standard for non-fungible tokens and defines the tokenURI function to provide metadata URI.

🔧 Debug

advanced

Why does this tokenURI function revert with 'URI query for nonexistent token' error?

Given this Solidity snippet, why does calling tokenURI(10) revert?

function tokenURI(uint256 tokenId) public view returns (string memory) {
    require(_exists(tokenId), "URI query for nonexistent token");
    return _tokenURIs[tokenId];
}

Blockchain / Solidity

mapping(uint256 => string) private _tokenURIs;

function _exists(uint256 tokenId) internal view returns (bool) {
    return _owners[tokenId] != address(0);
}

mapping(uint256 => address) private _owners;

A The _owners mapping is private and cannot be accessed causing revert.

B Token ID 10 has no owner, so _exists returns false causing require to fail.

C The function tokenURI is missing the override keyword causing revert.

D The _tokenURIs mapping is empty, so returning _tokenURIs[10] causes revert.

💡 Hint

Check what _exists function checks before returning URI.

The require statement checks if the token exists by verifying if it has an owner. If token 10 has no owner, _exists returns false and the require fails, reverting the call.

📝 Syntax

advanced

Which option correctly implements a tokenURI function with base URI and token ID?

Select the correct Solidity function that returns the token URI by concatenating a base URI and token ID.

A

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return string(abi.encodePacked(baseURI, tokenId));
}

B

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return baseURI + tokenId;
}

C

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return baseURI.concat(tokenId);
}

D

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return string(abi.encodePacked(baseURI, Strings.toString(tokenId)));
}

💡 Hint

Remember tokenId is a number and needs conversion to string before concatenation.

Option D correctly converts tokenId to string using Strings.toString and concatenates with baseURI using abi.encodePacked. Other options either try to concatenate string with uint directly or use invalid methods.

🚀 Application

expert

How many metadata JSON files are needed for an ERC-1155 contract with 3 token types?

An ERC-1155 contract manages 3 different token types with IDs 1, 2, and 3. Each token type has unique metadata. How many separate JSON metadata files should be hosted to fully support tokenURI calls?

A 1 JSON file containing metadata for all tokens combined

B 0 JSON files because ERC-1155 does not use metadata files

C 3 JSON files, one per token ID

D 6 JSON files, two per token ID for different formats

💡 Hint

Each token type in ERC-1155 can have its own metadata URI.

ERC-1155 tokens can have individual metadata URIs per token ID. For 3 token types, 3 separate JSON metadata files are needed, one for each token ID.