Object vs Function Defaults in Backbone.js: Which Approach Minimizes Shared‑State Bugs After an Upgrade?

When upgrading a Backbone.js application from an older release to 1.4+, the choice of defaults for Backbone.Model can affect data integrity. A static object is shared across instances, potentially causing unintended cross‑instance mutations. A function returns a fresh object per model, avoiding shared state but adding a small overhead. The unresolved decision is whether the safety of isolated defaults outweighs the performance cost in a large‑scale application, especially when legacy code may have relied on mutable defaults that could break after an upgrade.

Key constraints:

• Backbone 1.4+ supports both object and function defaults.
• Mutable defaults can silently corrupt data when one instance mutates a nested property.
• Function defaults add a minimal but measurable instantiation cost.
• Upgrade scripts must preserve the defaults mechanism to avoid unintended shared state.

Considering these factors, we need to evaluate the trade‑off between safety and performance. What are the measurable impacts of each approach on model instantiation time in high‑throughput scenarios? How does an upgrade from 1.3 to 1.4 affect models that originally used object defaults? Which strategy best balances data integrity and execution speed for large‑scale Backbone applications?