Implement a deepSealObject function

Apr 04, 2024

Object.seal() static method on the Object constructor when used on objects “seal” the object. A sealed object has a fixed set of properties: new properties cannot be added, existing properties cannot be removed, their enumerability and configurability cannot be changed, and its prototype cannot be re-assigned.

You can learn more about seal on MDN.

However, sealing only works on non-nested objects.

Variation: Deep Seal

1. Basic implementation

We have to implement a deepSealObject function which seals an object on every nested level.

We can verify whether an object is sealed or not using the Object.isSealed() static method on object constructor.

function deepSealObject(input) {
  const result = {};

  for (const key in input) {
    // only for non inherited keys
    if (Object.hasOwn(input, key)) {
      const value = input[key];

      if (typeof value !== 'object') {
        result[key] = Object.seal(value);
      } else {
        // recursively keep sealing each level
        result[key] = deepSealObject(value);
      }
    }
  }

  // finally seal the root
  return Object.seal(result);
}

// Usage
const input = {
  a: 1,
  b: {
    c: 2
  }
};

console.log(Object.isSealed(input)); // yields false

const output = deepSealObject(input);

console.log(Object.isSealed(output)); // yields true

delete output.b.c; // no effect

console.log(output);

Variation: Deep Freeze

2. Deep freeze implementation

We also have a Object.freeze() static method available which is very similar to Object.seal(). For an interview setting, one might be asked to implement deepFreezeObject as well.

The implementation remains exactly same except, we use the Object.freeze() method instead of Object.seal().

function deepFreezeObject(input) {
  const result = {};

  for (const key in input) {
    // only for non inherited keys
    if (Object.hasOwn(input, key)) {
      const value = input[key];

      if (typeof value !== 'object') {
        result[key] = Object.freeze(value);
      } else {
        // recursively keep sealing each level
        result[key] = deepFreezeObject(value);
      }
    }
  }

  // finally seal the root
  return Object.freeze(result);
}

Whether to deepFreeze or deepSeal an object really depends on what type of restrictions you want imposed on the object. It’s better to understand the difference between them, so as to make the best possible decision when it comes to preventing/restricting modifications of an object.

Implement a deepSealObject function

1. Basic implementation

2. Deep freeze implementation

Further Reading