Token metadata and URI in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is token metadata in blockchain?

Token metadata is information that describes a token's properties, such as its name, symbol, description, image, and other attributes that help users and applications understand the token.

beginner

What does URI stand for and what is its role in token metadata?

URI stands for Uniform Resource Identifier. In token metadata, it points to the location where the token's metadata JSON file is stored, often on IPFS or a web server.

intermediate

Why is storing token metadata off-chain common practice?

Because storing large amounts of data on the blockchain is expensive and inefficient, token metadata is usually stored off-chain (like on IPFS or centralized servers) and linked via a URI.

intermediate

What is a typical structure of a token metadata JSON file?

A typical token metadata JSON includes fields like 'name', 'description', 'image' (URL or IPFS hash), and sometimes 'attributes' or 'properties' describing traits of the token.

intermediate

How does a smart contract use the token URI?

The smart contract provides a function (like tokenURI in ERC-721) that returns the URI pointing to the token's metadata, allowing wallets and marketplaces to fetch and display token details.

What does the token URI usually point to?

A The token owner's wallet address

B The token's smart contract code

C A JSON file containing token metadata

D The blockchain network address

The token URI points to a JSON file that holds the token's metadata like name, image, and description.

Why is token metadata often stored off-chain?

A Because blockchain storage is costly and limited

B Because metadata is not important

C Because smart contracts cannot access metadata

D Because off-chain data is more secure

Storing metadata off-chain saves blockchain space and reduces costs.

Which of these is NOT typically part of token metadata JSON?

A Description

B Token name

C Image URL

D Token owner's private key

Private keys are secret and never included in token metadata.

What standard function returns the token URI in an ERC-721 contract?

A ownerOf

B tokenURI

C getMetadata

D transferFrom

The tokenURI function returns the URI pointing to the token's metadata.

Which protocol is commonly used to store token metadata off-chain?

A IPFS

B HTTP only

C FTP

D SMTP

IPFS is a decentralized storage protocol often used for token metadata.

Explain what token metadata is and why it is important in blockchain tokens.

Think about how metadata helps describe a token beyond just its ID.

Describe how a token URI works and how it connects a smart contract to token metadata.

Consider how the smart contract tells apps where to find token details.

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