String Delimiter Issues in JSON: Single vs. Double Quotes

One of the most common errors when working with JSON is using the wrong type of quotes for strings. Unlike many programming languages that accept either single or double quotes, JSON has a strict requirement: strings must be enclosed in double quotes only. This article explains why this restriction exists, how it causes problems, and how to avoid or fix these issues.

The JSON Specification and Quotes

According to the official JSON specification, strings in JSON must be enclosed in double quotes. This is a non-negotiable rule that affects both keys and values:

{
  "name": "John Doe",
  "age": 30,
  "preferences": {
    "theme": "dark",
    "notifications": true
  }
}

Common Errors with String Delimiters

1. Using Single Quotes for Keys or Values

Developers familiar with JavaScript, Python, or other languages often mistakenly use single quotes in JSON.

{
  'name': 'John Doe',
  'age': 30
}

2. Mixing Quote Types

Another common mistake is mixing quote types within the same JSON document.

{
  "name": 'John Doe',
  'age': 30
}

Why Only Double Quotes?

The JSON specification chose double quotes for several reasons:

How to Fix Quote Delimiter Issues

1. Manual Replacement

The simplest approach is to replace all single quotes with double quotes. Most text editors support find-and-replace operations with regular expressions:

2. Using JSON Formatter Tools

JSON formatter tools like the one provided by Offline Tools can automatically detect and repair quote delimiter issues. Simply paste your JSON with incorrect quotes, and the tool will attempt to fix the formatting.

3. Escaping Quotes in Strings

When you need to include double quotes inside a string in valid JSON, they must be escaped with a backslash:

{
  "message": "He said, \"Hello world!\"",
  "code": "console.log(\"testing\");"
}

Language-Specific Considerations

JavaScript

JavaScript allows single quotes for strings in code, but not when parsing JSON:

// This works in JavaScript
const obj = {
  'name': 'John'
};

// But this fails
const jsonString = '{"name": "John"}'; // Valid JSON string
const jsonString2 = "{'name': 'John'}"; // Invalid JSON string

// This will work
JSON.parse(jsonString); 

// This will throw an error
JSON.parse(jsonString2);

Python

Python developers need to be especially careful, as Python's dictionaries commonly use single quotes:

# Python dictionary with single quotes
python_dict = {
    'name': 'John',
    'age': 30
}

# Converting to valid JSON requires double quotes
import json
json_string = json.dumps(python_dict)  # Results in {"name": "John", "age": 30}