Safe JSON Stringification in JavaScript: fast-safe-stringify vs json-stringify-safe vs safe-json-stringify

When building robust frontend or Node.js applications, you’ll eventually need to serialize JavaScript objects that might contain circular references, functions, undefined, or other non-serializable values. Native JSON.stringify() throws an error on circular structures and silently drops undefined and functions — which can lead to unexpected behavior or crashes in production. The three packages under review—fast-safe-stringify, json-stringify-safe, and safe-json-stringify—all aim to solve this problem, but they differ significantly in performance, API design, maintenance status, and real-world reliability.

⚠️ Deprecation Status: One Package Is Officially Retired

Before diving into features, it’s critical to address project health:

• json-stringify-safe is officially deprecated. Its npm page states: "This module is no longer maintained. Please use fast-safe-stringify instead." It hasn’t seen a meaningful update since 2016 and contains known bugs with deeply nested circular structures.
• safe-json-stringify appears unmaintained. The GitHub repository has been archived, and the last npm publish was in 2017. No official deprecation notice exists, but active development has ceased.
• fast-safe-stringify is actively maintained, widely adopted in production systems (including by Pino logging), and regularly updated with performance improvements and bug fixes.

🛑 Bottom line: Avoid json-stringify-safe and safe-json-stringify in new projects. Only consider them for legacy compatibility.

🧪 Core Behavior: How Each Handles Problematic Values

All three libraries attempt to avoid throwing errors when encountering circular references or non-serializable types, but their strategies differ.

Circular References

fast-safe-stringify replaces circular references with the string "[Circular]".

// fast-safe-stringify
import stringify from 'fast-safe-stringify';

const obj = { a: 1 };
obj.self = obj;
console.log(stringify(obj));
// Output: "{\"a\":1,\"self\":\"[Circular]\"}"

json-stringify-safe also uses "[Circular]", but its implementation fails on complex graphs (e.g., multiple interlinked circular paths).

// json-stringify-safe
const stringifySafe = require('json-stringify-safe');

const obj = { a: 1 };
obj.self = obj;
console.log(stringifySafe(obj));
// Output: "{\"a\":1,\"self\":\"[Circular]\"}"

safe-json-stringify behaves similarly but uses a different internal tracking mechanism that can leak memory in long-running processes.

// safe-json-stringify
const safeStringify = require('safe-json-stringify');

const obj = { a: 1 };
obj.self = obj;
console.log(safeStringify(obj));
// Output: "{\"a\":1,\"self\":\"[Circular]\"}"

Functions, undefined, and Symbols

Native JSON.stringify() omits object properties with values of undefined, functions, or symbols. These libraries preserve structure by converting them to strings:

• fast-safe-stringify: Converts undefined → "undefined", function → "[Function: name]" or "[Function]", symbol → "[Symbol(name)]".

// fast-safe-stringify
const obj = {
  fn: () => {},
  undef: undefined,
  sym: Symbol('id')
};
console.log(stringify(obj));
// Output: "{\"fn\":\"[Function]\",\"undef\":\"undefined\",\"sym\":\"[Symbol(id)]\"}"

• json-stringify-safe: Converts all three to "[object Object]" or similar generic placeholders, losing semantic meaning.

// json-stringify-safe
console.log(stringifySafe(obj));
// Output: "{\"fn\":\"[object Object]\",\"undef\":\"[object Object]\",\"sym\":\"[object Object]\"}"

• safe-json-stringify: Similar to json-stringify-safe, uses vague placeholders like "[NonSerializable]".

// safe-json-stringify
console.log(safeStringify(obj));
// Output: "{\"fn\":\"[NonSerializable]\",\"undef\":\"[NonSerializable]\",\"sym\":\"[NonSerializable]\"}"

⚡ Performance: Speed Matters in Logging and Debugging

In high-throughput scenarios (e.g., logging middleware), serialization speed directly impacts application latency.

• fast-safe-stringify is optimized in C-like JavaScript with minimal allocations. Benchmarks consistently show it’s 2–5x faster than the alternatives.
• json-stringify-safe uses recursive traversal with heavy object copying, making it slow and memory-intensive.
• safe-json-stringify falls in between but still lags significantly behind fast-safe-stringify.

Example in a logging context:

// Using fast-safe-stringify in a logger
app.use((req, res, next) => {
  console.log('Request:', stringify(req)); // Fast, safe
  next();
});

Switching to json-stringify-safe here would noticeably degrade request throughput under load.

🔧 API Design: Simplicity vs Flexibility

All three expose a single function matching JSON.stringify(value, replacer?, space?), but only fast-safe-stringify supports the full signature reliably.

• fast-safe-stringify fully supports replacer and space arguments:

stringify({ a: 1, b: 2 }, ['a'], 2);
// Returns formatted string with only property 'a'

• json-stringify-safe accepts extra arguments but ignores them or behaves unpredictably.
• safe-json-stringify does not support replacer or space at all.

This makes fast-safe-stringify the only viable choice if you need pretty-printed output or custom property filtering.

🌐 Real-World Use Cases

✅ When You Need Reliable, High-Performance Serialization

Use fast-safe-stringify for:

• Application logging (e.g., with Pino, Winston)
• Debugging tools that inspect arbitrary objects
• Serializing user-provided data that may contain cycles
• Any production system where stability and speed are critical

⚠️ Legacy Code Only

Consider json-stringify-safe or safe-json-stringify only if:

• You’re maintaining an old codebase already dependent on them
• You cannot upgrade due to strict dependency policies
• You’ve verified their behavior matches your exact use case (not recommended)

📊 Summary Table

💡 Final Recommendation

For any new project requiring safe JSON serialization, fast-safe-stringify is the clear and only sensible choice. It’s fast, well-maintained, handles edge cases correctly, and integrates seamlessly into modern JavaScript toolchains. The other two packages exist only as historical artifacts — useful to recognize in legacy code, but never to adopt anew.