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

Use function defaults whenever a default value is mutable

For Backbone.js Model defaults, choose the function form whenever the default value contains an array, object, or other mutable structure. Object literals are safe only for flat scalar primitives (strings, numbers, booleans, null). This is the only reliable way to prevent cross-instance shared-state bugs after an upgrade.

Confirmed behavior

Backbone resolves defaults per instance. When defaults is a function, Backbone invokes it and uses the returned hash. When defaults is an object literal, Backbone copies the top-level keys into each new instance's attributes, but it does not deep-clone nested values. Two models created from the same class will share the same nested array or object reference unless the defaults function returns fresh values each call.

What the 1.3 → 1.4 upgrade changes

Do not assume that upgrading from Backbone 1.3 to 1.4 alters object-default sharing. The latent shared-reference behavior is the same. The upgrade matters only if changes in set, parse, validation, or event handling make an existing mutation visible. Inspect your bundled Backbone source to confirm how defaults is resolved in your build.

Performance: measure, don't guess

There is no universal instantiation-time number. Object defaults avoid one function call and one object allocation per model; function defaults add that cost. In most Backbone applications the overhead is negligible. If you are instantiating models at high throughput, benchmark with your real attributes, validation, and events in Node or the browser before deciding.

Recommended steps

1. Grep for defaults: object literals containing [] or {}.
2. Convert those to a function that returns a fresh hash each call.
3. Keep scalar-only defaults as object literals if you prefer.
4. Write a test that creates two instances, mutates one nested value via get(), and asserts the other instance is unchanged.
5. Run your upgrade test suite comparing attributes, changed, and validation before and after the upgrade.

const A = Backbone.Model.extend({
  defaults: () => ({ tags: [] })
});
const a = new A();
const b = new A();
a.get('tags').push('x');
console.assert(b.get('tags').length === 0, 'shared state detected');

What to avoid

• Returning a module-level constant or memoized object from the defaults function—this preserves shared state.
• Deep-cloning large structures inside defaults unless profiling shows instantiation is the bottleneck.
• Mutating model.get('nested') in place; Backbone does not protect nested attributes.

If your defaults function uses this or expects arguments, verify the exact _.result behavior in your Underscore/Lodash and Backbone build rather than relying on undocumented parameters.