How can I deep freeze an object in JavaScript?

Angelos Chalaris · Mar 17, 2020 ·

JavaScript, Object

Objects in JavaScript are mutable, regardless if you define them as const variables or not. In fact, using const when defining an object only prevents the variable from being reassigned. However, you can reassign the properties of a const object or array, like this:

const myObj = { a: 10, b: 20, c: 30 };
myObj.a = 12;     // { a: 12, b: 20, c: 30 };

const myArr = [15, 25, 35];
myArr[1] = 28;    // [15, 28, 35];

To make an object immutable, we can utilize Object.freeze(), which will prevent the addition of new properties and prevent deletion and changes to existing properties to some extent. However, while Object.freeze() provides somewhat of a solution, it only mitigates the problem to the next nesting level, as in reality it performs a shallow freeze. This means that properties that are objects or arrays can still be mutated:

const myObj = {
  a: 1,
  b: 'hello',
  c: [0, 1, 2],
  d: { e: 1, f: 2 }
};
Object.freeze(myObj);

myObj.a = 10;
myObj.b = 'hi';
myObj.c[1] = 4;
myObj.d.e = 0;
/*
myObj = {
  a: 1,
  b: 'hello',
  c: [0, 4, 2],
  d: { e: 0, f: 2 }
}
*/

As you can see, Object.freeze() is a step in the right direction, but only shallow freezes the object. To solve the issue we can use recursion, checking if each property is itself an object and, if Object.isFrozen() is false, apply Object.freeze() to it:

const myObj = {
  a: 1,
  b: 'hello',
  c: [0, 1, 2],
  d: { e: 1, f: 2 }
};

const deepFreeze = obj => {
  Object.keys(obj).forEach(prop => {
    if (typeof obj[prop] === 'object' && !Object.isFrozen(obj[prop])) deepFreeze(obj[prop]);
  });
  return Object.freeze(obj);
};
deepFreeze(myObj);

myObj.a = 10;
myObj.b = 'hi';
myObj.c[1] = 4;
myObj.d.e = 0;

/*
myObj = {
  a: 1,
  b: 'hello',
  c: [0, 1, 2],
  d: { e: 1, f: 2 }
}
*/

In the above example, we apply the techniques we described previously to ensure that the given object is deeply frozen. You can view the complete code, along with more examples in the deepFreeze snippet.

Image credit: Aaron Burden on Unsplash

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