SyntaxError: Unexpected token < in JSON at position 0
json.decoder.JSONDecodeError: Expecting value: line 1 column 1
JSON.parse: unexpected character at line 1 column 1

The string you’re trying to parse as JSON isn’t valid JSON.

Fix 1: Response Is HTML, Not JSON

// ❌ API returned an error page (HTML) instead of JSON
const res = await fetch('/api/data');
const data = await res.json();  // 💥 Unexpected token '<'

// ✅ Check response before parsing
const res = await fetch('/api/data');
const text = await res.text();
console.log(text);  // See what you actually got
const data = JSON.parse(text);

Fix 2: Empty Response

// ❌ Response body is empty
const data = JSON.parse('');  // 💥

// ✅ Check for empty response
const text = await res.text();
const data = text ? JSON.parse(text) : null;

Fix 3: Trailing Commas

// ❌ JSON doesn't allow trailing commas
{ "name": "Alice", "age": 30, }

// ✅ Remove trailing comma
{ "name": "Alice", "age": 30 }

Fix 4: Single Quotes

// ❌ JSON requires double quotes
{ 'name': 'Alice' }

// ✅ Use double quotes
{ "name": "Alice" }

Fix 5: Unescaped Characters

// ❌ Newlines and tabs must be escaped
{ "bio": "Line 1
Line 2" }

// ✅ Escape special characters
{ "bio": "Line 1\nLine 2" }

Fix 6: Python — Wrong Quotes in String

# ❌ Using Python dict syntax instead of JSON
import json
data = json.loads("{'name': 'Alice'}")  # 💥 Single quotes

# ✅ Use double quotes
data = json.loads('{"name": "Alice"}')

# Or convert Python dict to JSON
import json
data = {"name": "Alice"}
json_str = json.dumps(data)

Debugging

// Validate JSON before parsing
try {
    const data = JSON.parse(text);
} catch (e) {
    console.log('Invalid JSON:', text.substring(0, 200));
}

# Validate a JSON file
python3 -m json.tool file.json
# Or use jq
cat file.json | jq .

Related resources

• Unexpected token in JSON fix
• Unexpected end of JSON input fix
• Python JSON decode error fix