Lodash _.assign vs _.assignIn: Prototype Chains

In the Lodash JavaScript utility library, both _.assign and _.assignIn are used to copy properties from one or more source objects onto a target destination object. The fundamental difference between them lies in how they navigate prototype chains: _.assign copies only the source object's own enumerable properties, while _.assignIn copies both own and inherited enumerable properties found along the prototype chain. This distinction determines whether properties defined on a parent prototype will be transferred to your destination object.

Lodash _.assign

The _.assign method strictly copies an object's own enumerable string keyed properties to the destination object. It ignores any properties that the source object inherits from its prototype chain. This behavior is equivalent to the native ES6 Object.assign() method.

If a property exists on Source.prototype but is not directly assigned to the source instance itself, _.assign will omit it entirely.

const parent = { inheritedProp: 'from prototype' };
const child = Object.create(parent);
child.ownProp = 'from child';

const result = _.assign({}, child);

console.log(result.ownProp); // 'from child'
console.log(result.inheritedProp); // undefined

Lodash _.assignIn

The _.assignIn method (previously aliased as _.extend) copies both own and inherited enumerable properties from the source object to the destination object.

When _.assignIn executes, it walks up the prototype chain of the source object, identifies all enumerable properties, and assigns them as own properties directly onto the destination object.

const parent = { inheritedProp: 'from prototype' };
const child = Object.create(parent);
child.ownProp = 'from child';

const result = _.assignIn({}, child);

console.log(result.ownProp); // 'from child'
console.log(result.inheritedProp); // 'from prototype'
console.log(result.hasOwnProperty('inheritedProp')); // true

Direct Comparison

When to Use Which

Use _.assign for standard object merging, handling plain JavaScript objects (POJOs), and preventing unexpected inherited properties from polluting your destination object.

Use _.assignIn when working with custom class instances, prototype-based inheritance models, or when you explicitly need to flatten an inheritance structure into a single plain object containing all accessible properties.