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Node.js

JSON.stringify Error: Why Not Working & Solutions

By Filip on 10/05/2024

Learn why JSON.stringify doesn't work as expected with Error objects in Node.js and discover the correct way to serialize error data.

JSON.stringify Error: Why Not Working & Solutions

Table of Contents

Introduction

In JavaScript, you can't directly convert an Error object to a JSON string using JSON.stringify(errorObject). This is because Error objects have non-enumerable properties and might contain circular references, which JSON.stringify doesn't handle well. To get around this, you can either create a plain object with the error information you need or use a library like Lodash to create a deep copy of the error object without circular references. Let's explore both solutions.

Step-by-Step Guide

Let's break down why you can't directly stringify an Error object in JavaScript and how to get the information you need.

The Problem:

When you try JSON.stringify(errorObject), you often get an empty object {}. This happens because:

  1. Non-Enumerable Properties: Key properties of an Error object like message, name, and stack are marked as non-enumerable. JSON.stringify only includes enumerable properties by default.

  2. Circular References: Error objects might contain circular references (e.g., referencing other parts of the stack trace), which JSON.stringify can't handle directly.

Solutions:

Here's how to extract useful data from an Error for logging, debugging, or sending to a server:

1. Create a Plain Object:

function getErrorInfo(error) {
  return {
    message: error.message,
    name: error.name,
    stack: error.stack,
    // Add any other custom properties you need
  };
}

try {
  // ... your code that might throw an error
} catch (error) {
  const errorInfo = getErrorInfo(error);
  console.log(JSON.stringify(errorInfo)); 
}
  • Explanation: This approach manually copies the relevant properties from the Error object into a new plain object. Since the properties in the new object are enumerable by default, JSON.stringify works as expected.

2. Use a Library (for Deep Cloning and Circular References):

If you need to handle more complex error objects with potential circular references, consider using a library like Lodash:

const _ = require('lodash'); // Assuming you have Lodash installed

try {
  // ... your code
} catch (error) {
  const clonedError = _.cloneDeep(error); 
  const errorString = JSON.stringify(clonedError);
  console.log(errorString);
}
  • Explanation: Lodash's cloneDeep function creates a deep copy of the Error object, resolving circular references so that JSON.stringify can process it.

Important Considerations:

  • Security: Be mindful of what error information you expose, especially in production environments. Avoid leaking sensitive data in error messages.
  • Context: Along with the error itself, include relevant context information (e.g., user input, timestamps, affected data) to aid in debugging.

Code Example

This JavaScript code demonstrates how to handle stringifying Error objects. It first shows that directly stringifying an Error object can fail. Then, it provides two solutions: manually extracting error properties into a plain object and using Lodash's cloneDeepWith to handle potential circular references. The code emphasizes the importance of providing context in error information and being mindful of security risks.

// **1. The Problem:** Demonstrating why direct stringify fails

const myError = new Error('Something went wrong!');

try {
  JSON.stringify(myError); 
} catch (e) {
  console.log('Direct stringify failed:', e); 
}

// **2. Solution 1: Creating a Plain Object**

function getErrorInfo(error) {
  return {
    message: error.message,
    name: error.name,
    stack: error.stack,
    time: new Date() // Adding a timestamp for context
  };
}

try {
  throw new Error('A custom error occurred!');
} catch (error) {
  const errorInfo = getErrorInfo(error);
  console.log('Extracted error info:', JSON.stringify(errorInfo, null, 2)); 
}

// **2. Solution 2: Using Lodash for Deep Cloning (if needed)**

const _ = require('lodash'); 

function createCircularError() {
  const error = new Error('Circular reference!');
  error.relatedError = error; // Creating a circular reference
  return error;
}

try {
  throw createCircularError();
} catch (error) {
  const clonedError = _.cloneDeepWith(error, (value) => {
    if (value instanceof Error) {
      return { message: value.message, name: value.name, stack: value.stack };
    }
  });
  const errorString = JSON.stringify(clonedError, null, 2);
  console.log('Cloned error:', errorString);
}

Explanation:

  1. The Problem: The code first shows that directly stringifying an Error object often results in an error or an empty object.
  2. Solution 1: The getErrorInfo function demonstrates how to manually extract the desired properties into a new object, making it stringify-friendly.
  3. Solution 2: This part uses Lodash's cloneDeepWith to handle potential circular references within the Error object. It creates a deep copy of the error object, replacing any nested Error instances with their stringified representations to avoid circularity.

Key Points:

  • Context is Crucial: Notice how the example adds a timestamp to the error information. Providing context like this is extremely helpful for debugging.
  • Security First: In real-world applications, be very careful about what information you include in error logs, especially in production. Avoid exposing sensitive data.
  • Choose the Right Tool: For simple cases, manually creating a plain object is sufficient. For complex errors with potential circular references, a library like Lodash provides a robust solution.

Additional Notes

  • Enumerable vs. Non-Enumerable Properties: Understanding the difference between enumerable and non-enumerable properties in JavaScript is crucial for working with JSON.stringify. Enumerable properties are included in for...in loops and by JSON.stringify by default. You can use Object.getOwnPropertyDescriptors(errorObject) to see which properties are enumerable.

  • Alternatives to Libraries: While libraries like Lodash offer a convenient way to deep clone, you can achieve similar results with custom recursive functions if you want to avoid external dependencies.

  • Error Handling Best Practices: This article focuses on stringifying errors, but remember that good error handling involves more than just logging. Consider:

    • User-Friendly Messages: Displaying raw error messages to users can be confusing. Provide clear and helpful explanations instead.
    • Error Boundaries (in UI Frameworks): Use error boundaries in front-end frameworks like React to prevent the entire application from crashing due to a single error.
    • Centralized Error Logging: In production, log errors to a centralized service (e.g., Sentry, LogRocket) for better monitoring and analysis.

  • JSON.stringify Replacer Function: For more fine-grained control over the stringification process, you can provide a replacer function as the second argument to JSON.stringify. This function allows you to transform values before they are stringified.

  • Security Implications of Stack Traces: Stack traces can sometimes reveal internal file paths or other sensitive information. Be cautious about exposing full stack traces in production environments, especially to clients.

Summary

Problem Description Solution
Non-Enumerable Properties Key error properties (message, name, stack) are not included in JSON.stringify by default. 1. Create a Plain Object: Manually copy desired properties into a new object.
2. Use a Library: Utilize libraries like Lodash's cloneDeep to handle deep cloning and circular references.
Circular References Error objects might contain circular references, causing JSON.stringify to fail. (See solution for "Non-Enumerable Properties")

Code Example (Plain Object):

function getErrorInfo(error) {
  return {
    message: error.message,
    name: error.name,
    stack: error.stack,
    // Add custom properties as needed
  };
}

try {
  // ... code that might throw an error
} catch (error) {
  const errorInfo = getErrorInfo(error);
  console.log(JSON.stringify(errorInfo)); 
}

Important Considerations:

  • Security: Avoid exposing sensitive data in error messages.
  • Context: Include relevant context information for better debugging.

Conclusion

In conclusion, directly stringifying JavaScript Error objects using JSON.stringify often leads to unexpected results due to non-enumerable properties and potential circular references within these objects. To overcome this, developers can opt for two primary solutions: manually extracting relevant error information into a new, plain object or employing libraries like Lodash to create deep copies of the error object while handling circular dependencies. When choosing a solution, it's essential to prioritize security by carefully considering the information included in error logs, especially in production environments. Additionally, providing context alongside error data significantly aids in debugging and troubleshooting. By understanding these nuances and adopting appropriate techniques, developers can effectively manage and utilize error information in their JavaScript applications.

References

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