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Guide to Handling Javascript TypeError: Solving the Readonly Property Issue

Karthik MSN
~ 3 min read | Published on Feb 28, 2024


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Welcome to a deep dive into one of the common stumbling blocks in the JavaScript world: the TypeError (readonly property). Whether you're a seasoned developer or someone just getting your feet wet in JavaScript, understanding and handling this error is crucial for robust application development. In this article, we'll unravel the mysteries of TypeError (readonly property) in JavaScript, providing you with insights and practical solutions to avoid and fix this error. Our journey will include real-life code scenarios, detailed examples, and how tools like Zipy can be instrumental in debugging.

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Understanding TypeError (readonly property) in JavaScript

A TypeError (readonly property) occurs when an attempt is made to modify a property that has been defined as read-only. This can happen in several contexts, such as when dealing with native JavaScript objects, using libraries or frameworks that define read-only properties, or working within strict mode which enforces immutability more aggressively. Understanding why and how this error occurs is the first step in preventing it from derailing your development process.

Scenario 1

Error code

const person = Object.freeze({ name: "John" }); = "Jane"; // Attempt to modify a read-only property

Corrected code

const person = { name: "John" }; // Removed Object.freeze to allow modifications = "Jane"; // This line now works as expected

Solution Summary

In this scenario, the TypeError (readonly property) was caused by the use of Object.freeze(), which makes an object immutable. The solution was to remove Object.freeze() to allow property modifications.

Scenario 2

Error code

Object.defineProperty(window, "PI", { value: 3.14, writable: false });
window.PI = 3.14159; // Attempt to overwrite a read-only property

Corrected code

Object.defineProperty(window, "PI", { value: 3.14, writable: true }); // Made the property writable
window.PI = 3.14159; // This line now updates the property as expected

Solution Summary

This scenario involved a global property defined with Object.defineProperty() as read-only. Changing the writable attribute to true in the property descriptor allows the property to be overwritten.

Scenario 3

Error code

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;

const square = new Rectangle(10, 10);
square.width = 20; // Attempt to modify a read-only property

Corrected code

class Rectangle {
  constructor(width, height) {
    this.width = width;
    this.height = height;
    // Removed Object.freeze to allow modifications

const square = new Rectangle(10, 10);
square.width = 20; // This line now works as expected

Solution Summary

Here, the error was triggered by freezing the Rectangle object instance, preventing any modifications. The solution was simply not to freeze the object, allowing its properties to be mutable.

Handling TypeError (readonly property) in Javascript

Avoiding TypeError (readonly property) in JavaScript requires a deep understanding of how properties are defined and modified. Using Object.defineProperty() and Object.freeze() with clear intent and understanding their implications is crucial. Always verify the property descriptors like writable, configurable, and others before attempting to modify an object's properties.

Proactive Error Debugging with Zipy

In the quest for bug-free code, tools like Zipy offer a game-changing approach. Zipy's proactive error monitoring and user session replay capabilities enable developers to catch and debug runtime JavaScript errors in real-time, providing insights that go beyond the code to include user interaction patterns. Incorporating Zipy into your development workflow can significantly reduce debugging time and improve code quality.

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Understanding and resolving TypeError (readonly property) in JavaScript is essential for developing dynamic, error-free applications. Through the examples and solutions provided, we've seen how to identify and correct this error, enhancing our code's robustness. Remember, proactive error debugging with tools like Zipy can further empower your development process, making error resolution more efficient and intuitive.

Resources on how to debug and fix Javascript Errors

Frequently Asked Questions

Why does TypeError (readonly property) occur in JavaScript?

TypeError (readonly property) occurs when code attempts to modify a property that has been set as immutable. This could be due to using Object.freeze(), defining properties with Object.defineProperty() without setting writable: true, or modifying properties of objects within a strict mode context.

How can I prevent TypeError (readonly property) in my projects?

Preventing TypeError (readonly property) involves careful design and coding practices. Ensure properties are not inadvertently set as read-only unless required,

and use Object.defineProperty() judiciously. Consider the use of development tools and error monitoring solutions like Zipy for early detection and debugging.

Can I modify a read-only property in JavaScript?

Directly modifying a read-only property is not possible. However, you can redefine the property using Object.defineProperty() if the property descriptor allows for it (i.e., if it's configurable). Always review the property's descriptors before attempting modifications.

What are the best practices for using Object.freeze() in JavaScript?

Use Object.freeze() when you want to ensure the integrity of an object's state. It's particularly useful in functional programming or when working with constants. However, remember that freezing an object is shallow, affecting only the immediate properties of the object.

How does Zipy help in debugging TypeError (readonly property)?

Zipy aids in debugging by providing real-time error monitoring and user session replay, offering context around the error occurrence. This insight allows developers to understand the circumstances leading to the error, making it easier to identify and rectify the issue efficiently.

Key takeaways

  • Understanding and handling TypeError (readonly property) in JavaScript is crucial for writing robust applications.
  • Proper use of Object.freeze() and Object.defineProperty() can prevent unintended read-only properties.
  • Debugging tools like Zipy enhance error resolution by providing valuable context and real-time monitoring capabilities.
  • Always review and understand property descriptors before attempting to modify object properties to avoid TypeErrors.

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