Lodash assignIn and Prototype Chain Inheritance

This article examines the exact mechanics of Lodash’s _.assignIn (also known as _.extend) when transferring properties from deeply nested prototype chains to a target object. It details the specific categories of properties that are copied—such as enumerable string-keyed properties—as well as those that are excluded, including Symbols, non-enumerable descriptors, and uninvoked accessors.

Enumerable String-Keyed Properties

_.assignIn iterates over both own and inherited properties of source objects. Across a deep prototype chain, it transfers only enumerable, string-keyed properties.

Under the hood, _.assignIn uses Lodash's internal keysIn operation, which mimics the behavior of a standard JavaScript for...in loop. Any property defined along the prototype chain that has its enumerable attribute set to true is extracted and copied to the target object as an own property.

Excluded Properties

When traversing prototype chains, _.assignIn deliberately ignores several types of properties:

• Non-Enumerable Properties: Built-in methods on Object.prototype (such as toString, valueOf, and hasOwnProperty) and custom methods or fields defined via Object.defineProperty with enumerable: false are skipped.
• Symbol-Keyed Properties: Inherited or own properties keyed by ES6 Symbols are not transferred. Lodash restricts _.assignIn strictly to string keys.
• Prototype References: The actual prototype linkages (__proto__) are not copied. The target object does not adopt the prototype chain of the source; instead, the inherited properties are flattened into own properties on the target.

Prototype Shadowing and Resolution Order

When the same property name exists at multiple levels of a deep prototype chain, _.assignIn honors JavaScript's standard prototype shadowing rules:

1. Properties found closest to the source instance take precedence over properties higher up in the chain.
2. If an instance defines an own property x, and its prototype also defines x, the value on the instance is the one assigned to the target.
3. If multiple source objects are supplied to _.assignIn(target, source1, source2), subsequent sources overwrite properties set by earlier sources.

Value Evaluation and Accessors

_.assignIn performs an assignment ([[Set]]) rather than a property descriptor definition ([[DefineOwnProperty]]):

• Getters: If an inherited property is configured as an accessor descriptor (a getter), _.assignIn invokes the getter during iteration and assigns the resulting return value to the target as a plain data property. The getter function itself is not copied.
• Setters: If the target object has a setter for a matching property name, the transferred value will trigger that setter on the target.