JavaScript Object seal() - Make Object Non-Extensible

Introduction

The Object.seal() method in JavaScript is a robust tool for controlling object modification. It allows you to seal an object, preventing new properties from being added to it while also marking all existing properties as non-configurable. Properties of a sealed object can still be changed as long as they are writable. This functionality is particularly useful in maintaining object integrity, avoiding accidental modifications or expansions after an object's initial creation and setup.

In this article, you will learn how to effectively use the Object.seal() method to make objects non-extensible. Discover how to seal objects, check if an object is sealed, and understand the implications of sealing objects in your JavaScript projects.

Understanding Object.seal()

Sealing an Object

1. Start with a simple object.

2. Apply the Object.seal() function to restrict it.

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   const user = { name: "Alex", age: 30 };
   Object.seal(user);
   

   This code creaes an object user and seals it using Object.seal(). After sealing, you cannot add new properties to the user object, nor can you remove existing ones.

Modifying Properties After Sealing

1. Despite sealing, modify the values of existing writable properties.

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   user.age = 31;
   console.log(user.age);  // Outputs: 31
   

   This example demonstrates modifying the age property of the sealed user object. The change is allowed because the existing properties remain writable.

Attempting to Extend the Sealed Object

1. Try to add a new property to the sealed object and observe the result.

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   user.location = "New York";
   console.log(user.location);  // Outputs: undefined
   

   Attempting to add a new property location to the sealed user object does not work, and accessing user.location returns undefined since the addition is ignored.

Checking if an Object is Sealed

Utilizing Object.isSealed()

1. Verify whether an object has been sealed.

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   console.log(Object.isSealed(user));  // Outputs: true
   

   The Object.isSealed() method checks if the object user is sealed, returning true if it is sealed, and false otherwise. This method is helpful for debugging and enforcing object integrity during runtime.

Implications of Sealing Objects

Understand the Key Effects of Sealing

• Prevents adding new properties.
• Sets all existing properties’ configurations to non-configurable.

Consider Use Cases

• Use Object.seal() when you want to lock an object's structure after its initial setup.
• Use sealing to prevent future extensions or structural changes, especially when working with settings or configurations that must not be altered after loading.

Conclusion

The Object.seal() method is a valuable tool for managing object mutability in JavaScript. By preventing extensions and making object properties non-configurable, you help ensure the stability and integrity of your objects throughout their lifecycle. Use Object.seal() to maintain predictable object behavior, guard against inadvertent property additions or deletions, and uphold object structure as you develop and manage your JavaScript applications. Implementing these techniques secures your objects against modifications that could potentially disrupt your application’s state or behavior.