Token metadata and URI in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the token URI variable.

Blockchain / Solidity

string private [1];

Drag options to blanks, or click blank then click option'

Auri

Bmetadata

CtokenURI

DtokenId

Attempts:

3 left

💡 Hint

Common Mistakes

Using tokenId as the URI variable name

Using metadata which is too generic

Using tokenURI which is usually a function, not a variable

The token URI is usually stored in a variable named uri or similar. Here, uri is the private string holding the metadata URI.

2fill in blank

medium

Complete the function signature to return the token URI.

Blockchain / Solidity

function [1](uint256 tokenId) public view returns (string memory) {

Drag options to blanks, or click blank then click option'

AgetTokenURI

BtokenURI

Curi

DmetadataURI

Attempts:

3 left

💡 Hint

Common Mistakes

Using getTokenURI which is not standard

Using uri which is usually a variable

Using metadataURI which is uncommon

The standard function to get a token's metadata URI in ERC721 is tokenURI.

3fill in blank

hard

Fix the error in the return statement to correctly return the token URI.

Blockchain / Solidity

return [1][tokenId];

Drag options to blanks, or click blank then click option'

Auri

BtokenURI

Cmetadata

DtokenId

Attempts:

3 left

💡 Hint

Common Mistakes

Returning tokenURI which is a function

Returning tokenId which is a number, not a URI

Returning metadata which is undefined

The URI is stored in a mapping or variable named uri, so we return uri[tokenId].

4fill in blank

hard

Fill both blanks to declare and initialize the mapping for token URIs.

Blockchain / Solidity

mapping(uint256 => [1]) private [2];

Drag options to blanks, or click blank then click option'

Astring

Buint256

Curi

DtokenURI

Attempts:

3 left

💡 Hint

Common Mistakes

Using uint256 as the value type

Using tokenURI as the mapping name

Using uint256 as the value type instead of string

The mapping stores a string URI for each token ID. The mapping name is uri.

5fill in blank

hard

Fill all three blanks to set the token URI in the function.

Blockchain / Solidity

function setTokenURI(uint256 tokenId, [1] memory [2]) public {
    [3][tokenId] = [2];
}

Drag options to blanks, or click blank then click option'

Astring

Buri

DtokenURI

Attempts:

3 left

💡 Hint

Common Mistakes

Using tokenURI as parameter name

Using uint256 as parameter type

Using different names for parameter and mapping

The function takes a string memory uri parameter and sets uri[tokenId] to that value.

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