next.js/packages/next/src/shared/lib/deep-freeze.ts

import type { DeepReadonly } from './deep-readonly'

/**
 * Recursively freezes an object and all of its properties. This prevents the
 * object from being modified at runtime. When the JS runtime is running in
 * strict mode, any attempts to modify a frozen object will throw an error.
 *
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze
 * @param obj The object to freeze.
 */
export function deepFreeze<T extends object>(obj: T): DeepReadonly<T> {
  // If the object is already frozen, there's no need to freeze it again.
  if (Object.isFrozen(obj)) return obj as DeepReadonly<T>

  // An array is an object, but we also want to freeze each element in the array
  // as well.
  if (Array.isArray(obj)) {
    for (const item of obj) {
      if (!item || typeof item !== 'object') continue
      deepFreeze(item)
    }

    return Object.freeze(obj) as DeepReadonly<T>
  }

  for (const value of Object.values(obj)) {
    if (!value || typeof value !== 'object') continue
    deepFreeze(value)
  }

  return Object.freeze(obj) as DeepReadonly<T>
}