如何修复Moralis API获取的Token元数据中metadata字段JSON格式异常问题

Problem Description #

I wrote code to fetch data from an API and save it to a file: using Moralis.Web3API.token.getTokenIdMetadata to retrieve token metadata. Since the returned object has a lot of unnecessary information, I extract the required fields into a new tokenMetadata object and save it to a file. Here's the code:

const tokenIdMetadata = await Moralis.Web3API.token.getTokenIdMetadata(options);
const tokenMetadata = {};
tokenMetadata.token_id = tokenIdMetadata.token_id;
tokenMetadata.metadata = tokenIdMetadata.metadata;
fs.writeFileSync(folderName.concat('/', collectionSlug, '_', tokenID, '.json'), JSON.stringify(tokenMetadata, null, 4));

The issue is that the metadata field in the generated JSON file is stored as an escaped string, which doesn't match the expected format:

"token_id": "4",
"metadata": "{\"name\":\"Arcade Land Mega #4\",\"image\":\"ipfs://QmaJyxmwWkH17P7cpzRDFqcQwh7HYZ53VS9pJN3rtWKVPh\",\"description\":\"Arcade Lands are digital properties in an interoperable metaverse that serves as your web3 home for all your NFTs. Land owners will get additional benefits within the Arcadeverse.\",\"attributes\":[{\"trait_type\":\"Size\",\"value\":\"Mega\"},{\"trait_type\":\"Area\",\"value\":\"Frozen\"},{\"trait_type\":\"Underground\",\"value\":\"Weird Rocks\"}]}"

The expected format should have metadata as a nested JSON object:

"token_id" : "4",
"metadata" : {
  "name": "Arcade Land Mega #4",
  "image": "ipfs://QmXYYbVEFqVPRELzPN3fgCw9v3wr2iQqixbVaGS1JTH6mS",
  "description": "Arcade Lands are digital properties in an interoperable metaverse that serves as your web3 home for all your NFTs. Land owners will get additional benefits within the Arcadeverse.",
  "attributes": [
    { "trait_type": "Size", "value": "Mega" },
    { "trait_type": "Area", "value": "Frozen" },
    { "trait_type": "Underground", "value": "Weird Rocks" }
  ]
}

How can I modify the code to fix this formatting issue?

Solution #

The core problem here is that the metadata value returned by the Moralis API is a JSON-encoded string, not a native JavaScript object. When you pass this string directly to JSON.stringify(), it gets escaped instead of being serialized as a nested object.

To fix this, you need to parse the string into a JavaScript object using JSON.parse() before assigning it to tokenMetadata.metadata. Here's the updated code:

const tokenIdMetadata = await Moralis.Web3API.token.getTokenIdMetadata(options);
const tokenMetadata = {};
tokenMetadata.token_id = tokenIdMetadata.token_id;
// Parse the JSON string into an object
tokenMetadata.metadata = JSON.parse(tokenIdMetadata.metadata);
fs.writeFileSync(`${folderName}/${collectionSlug}_${tokenID}.json`, JSON.stringify(tokenMetadata, null, 4));

Important Note: Handle Potential Parsing Errors #

Sometimes, NFT metadata strings might be malformed or invalid JSON. To avoid your code crashing when this happens, wrap the parsing step in a try-catch block:

const tokenIdMetadata = await Moralis.Web3API.token.getTokenIdMetadata(options);
const tokenMetadata = {};
tokenMetadata.token_id = tokenIdMetadata.token_id;

try {
  tokenMetadata.metadata = JSON.parse(tokenIdMetadata.metadata);
} catch (error) {
  console.error(`Failed to parse metadata for token ID ${tokenID}:`, error);
  // Optionally fall back to the original string or skip saving this token
  tokenMetadata.metadata = tokenIdMetadata.metadata;
}

fs.writeFileSync(`${folderName}/${collectionSlug}_${tokenID}.json`, JSON.stringify(tokenMetadata, null, 4));

This way, even if the metadata is invalid, your code will continue to run and you'll get a clear error message to debug.

内容的提问来源于stack exchange，提问作者Ben