10 JSON Mistakes That Will Break Your API (And How to Fix Them)

#webdev #ai #json #javascript

You paste JSON into your code. Everything looks fine. Then: SyntaxError: Unexpected token
Here are the 10 most common JSON mistakes I see (and how to fix them in seconds)."
Mistake #1: Trailing Commas

json// ❌ Wrong
{
  "name": "John",
  "age": 25,  ← trailing comma breaks JSON
}

// ✅ Correct
{
  "name": "John",
  "age": 25
}

Why it breaks: JSON spec doesn't allow trailing commas
How to fix: Remove the last comma
Pro tip: Use a JSON validator to catch this
Mistake #2: Single Quotes Instead of Double Quotes

json// ❌ Wrong
{'name': 'John'}

// ✅ Correct
{"name": "John"}
Mistake #3: Comments in JSON
json// ❌ Wrong
{
  // This is a user object
  "name": "John"
}

// ✅ Correct (JSON doesn't support comments)
{
  "_comment": "This is a user object",
  "name": "John"
}

Mistake #4: Unquoted Keys

json// ❌ Wrong
{name: "John"}

// ✅ Correct
{"name": "John"}
Mistake #5: Using undefined or NaN
json// ❌ Wrong
{"age": undefined, "score": NaN}

// ✅ Correct
{"age": null, "score": null}
Mistake #6: Multi-line Strings Without Escaping
json// ❌ Wrong
{
  "bio": "This is a
  multi-line string"
}

// ✅ Correct
{
  "bio": "This is a\nmulti-line string"
}
Mistake #7: Mixing Data Types Inconsistently
json// ❌ Confusing
{
  "users": [
    {"id": 1, "name": "John"},
    {"id": "2", "name": "Jane"}  ← ID is string here
  ]
}

// ✅ Consistent
{
  "users": [
    {"id": 1, "name": "John"},
    {"id": 2, "name": "Jane"}
  ]
}

Mistake #8: Circular References

javascript// ❌ This crashes
const obj = {};
obj.self = obj;
JSON.stringify(obj); // Error!

// ✅ Avoid circular references
const obj = { name: "John" };
JSON.stringify(obj); // Works

Mistake #9: Special Characters Without Escaping

json// ❌ Wrong
{"path": "C:\Users\John"}

// ✅ Correct
{"path": "C:\\Users\\John"}

Mistake #10: Wrong MIME Type in API Responses

javascript// ❌ Wrong
res.setHeader('Content-Type', 'text/plain');
res.send(JSON.stringify(data));

// ✅ Correct
res.setHeader('Content-Type', 'application/json');
res.json(data);

How to Avoid These Mistakes
1. Use a JSON validator
Before using JSON, validate it. Tools like [jsontoall.tools/json-validator] catch these errors instantly.
2. Use JSON.parse() in try-catch

javascripttry {
  const data = JSON.parse(jsonString);
} catch (e) {
  console.error('Invalid JSON:', e.message);
}

3. Use a linter
ESLint can catch JSON issues in your code.
4. Test with sample data
Always test your JSON with edge cases.
Conclusion
"These 10 mistakes account for 90% of JSON errors I see. Bookmark this guide - you'll need it.
Need to validate JSON quickly? Try [jsontoall.tools] - instant validation, clear error messages, free."

DEV Community

10 JSON Mistakes That Will Break Your API (And How to Fix Them)

Top comments (0)