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.

Practice

1. What is the main purpose of token metadata in blockchain tokens?

easy

A. To mine new tokens automatically

B. To describe the token and link to its details

C. To execute smart contract functions

D. To store the token's private key

Solution

1. Step 1: Understand token metadata role

   Token metadata provides descriptive information about the token, such as its name, description, and image link.

2. Step 2: Differentiate from other blockchain functions

   Metadata is not about private keys, mining, or executing contracts but about describing the token for users and apps.

3. Final Answer:

   To describe the token and link to its details -> Option B

4. Quick Check:

   Token metadata = description and details link [OK]

Hint: Metadata describes tokens, not keys or mining [OK]

Common Mistakes:

• Confusing metadata with private keys
• Thinking metadata controls token creation
• Mixing metadata with smart contract code

2. Which of the following is the correct way to specify a token URI in a smart contract?

easy

A. bool tokenURI = true;

B. int tokenURI = 12345;

C. string public tokenURI = "https://example.com/token/1.json";

D. address tokenURI = 0x123;

Solution

1. Step 1: Identify the data type for URI

   A URI is a web address, so it should be stored as a string in the contract.

2. Step 2: Check the options for correct syntax

   string public tokenURI = "https://example.com/token/1.json"; uses a string with a valid URL format, while others use wrong types like int, bool, or address.

3. Final Answer:

   string public tokenURI = "https://example.com/token/1.json"; -> Option C

4. Quick Check:

   URI is a string URL [OK]

Hint: URIs are strings, not numbers or booleans [OK]

Common Mistakes:

• Using int or bool for URI
• Confusing address type with URI
• Missing quotes around the URI string

3. Given the following Solidity snippet, what will tokenURI(1) return if _tokenURIs[1] is set to "https://example.com/metadata/1.json"?

mapping(uint256 => string) private _tokenURIs;

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return _tokenURIs[tokenId];
}

medium

A. An error because tokenURI is private

B. tokenId

C. Empty string

D. "https://example.com/metadata/1.json"

Solution

1. Step 1: Understand the mapping and function

   The mapping stores strings for each tokenId. The function returns the string stored for the given tokenId.

2. Step 2: Check the value for tokenId 1

   Since _tokenURIs[1] is set to "https://example.com/metadata/1.json", calling tokenURI(1) returns that string.

3. Final Answer:

   "https://example.com/metadata/1.json" -> Option D

4. Quick Check:

   Mapping lookup returns stored URI string [OK]

Hint: Mapping returns stored URI for tokenId [OK]

Common Mistakes:

• Thinking tokenURI is private and inaccessible
• Confusing tokenId with URI string
• Assuming function returns tokenId instead of URI

4. Identify the error in this Solidity function that tries to set a token URI:

mapping(uint256 => string) private _tokenURIs;

function setTokenURI(uint256 tokenId, string memory uri) public {
    _tokenURIs[tokenId] = uri;
}

medium

A. The function lacks access control and allows anyone to set URIs

B. The mapping type is incorrect for storing strings

C. The function should return a string

D. The parameter uri should be declared as calldata

Solution

1. Step 1: Review function purpose and security

   The function sets a URI for a tokenId but has no restriction on who can call it.

2. Step 2: Identify missing access control

   Without access control (like onlyOwner), anyone can change token URIs, which is a security risk.

3. Final Answer:

   The function lacks access control and allows anyone to set URIs -> Option A

4. Quick Check:

   Missing access control = security risk [OK]

Hint: Always add access control to setter functions [OK]

Common Mistakes:

• Thinking mapping type is wrong for strings
• Expecting setter functions to return values
• Ignoring security risks of public setters

5. You want to create a function that returns a JSON metadata URI for a token, but if the token has no specific URI set, it should return a default URI. Which Solidity code snippet correctly implements this logic?

mapping(uint256 => string) private _tokenURIs;
string private _defaultURI = "https://example.com/default.json";

function tokenURI(uint256 tokenId) public view returns (string memory) {
    // Fill in the blank
}

hard

A. return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI;

B. if (_tokenURIs[tokenId] == null) { return _defaultURI; } else { return _tokenURIs[tokenId]; }

C. return _tokenURIs[tokenId] || _defaultURI;

D. return _tokenURIs[tokenId] == "" ? _tokenURIs[tokenId] : _defaultURI;

Solution

1. Step 1: Check how to test if string is empty in Solidity

   Solidity strings can't be compared to null or use || operator; use bytes length to check emptiness.

2. Step 2: Verify correct conditional expression

   return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI; uses bytes(_tokenURIs[tokenId]).length > 0 to check if URI exists, else returns default URI.

3. Final Answer:

   return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI; -> Option A

4. Quick Check:

   Check string length with bytes() for empty [OK]

Hint: Use bytes().length to check empty string in Solidity [OK]

Common Mistakes:

• Using null or empty string comparison directly
• Using || operator for strings
• Returning wrong URI when none set