Token metadata and URI in Blockchain / Solidity

Introduction

Token metadata and URI help describe what a token is and where to find its details. This makes tokens easy to understand and use.

When creating a new digital token like an NFT or cryptocurrency.

When you want to show details about a token on a website or app.

When you need to link a token to images, descriptions, or other info.

When building marketplaces that display token information.

When verifying token authenticity and properties.

Syntax

Blockchain / Solidity

tokenURI = "https://example.com/api/token/123"

metadata = {
  "name": "Token Name",
  "description": "What this token represents",
  "image": "https://example.com/images/token123.png",
  "attributes": [
    {"trait_type": "Color", "value": "Red"},
    {"trait_type": "Size", "value": "Large"}
  ]
}

The tokenURI is a link to a JSON file with token details.

Metadata is usually a JSON object describing the token's properties.

Examples

This URI points to the metadata JSON for token #1.

Blockchain / Solidity

tokenURI = "https://mytokens.com/metadata/1.json"

Basic metadata with name, description, and image link.

Blockchain / Solidity

{
  "name": "Cool NFT",
  "description": "A unique digital art piece.",
  "image": "https://images.com/coolnft.png"
}

Metadata with custom attributes to describe token traits.

Blockchain / Solidity

{
  "name": "Rare Token",
  "attributes": [
    {"trait_type": "Rarity", "value": "Legendary"},
    {"trait_type": "Power", "value": 9000}
  ]
}

Sample Program

This program creates a token object with an ID and base URI. It then prints the full URI where the token's metadata JSON can be found.

Blockchain / Solidity

class Token:
    def __init__(self, token_id, base_uri):
        self.token_id = token_id
        self.base_uri = base_uri

    def token_uri(self):
        return f"{self.base_uri}/{self.token_id}.json"

# Create a token with ID 42 and base URI
my_token = Token(42, "https://tokens.example.com/metadata")

# Print the full URI to the token metadata
print(my_token.token_uri())

Important Notes

Token metadata is often stored off-chain (outside the blockchain) to save space.

URIs usually point to web servers or decentralized storage like IPFS.

Always ensure metadata links are stable and accessible to avoid broken tokens.

Summary

Token metadata describes what a token is and its details.

Token URI is a link to the metadata JSON file.

Metadata helps apps and users understand and display tokens properly.

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