objectmerging

Object merging refers to the process of combining two or more objects into a single object by taking properties from source objects and applying them to a target object. The merge typically resolves conflicting keys by favoring properties from later sources. There are two common flavors: shallow merge, which copies enumerable own properties at one level; and deep (recursive) merge, which traverses nested objects to merge their properties.

In many programming environments, merges can be performed mutating the target or by returning a new object. In JavaScript, common patterns include shallow merges with Object.assign({}, a, b) or the spread syntax { ...a, ...b }, both producing a new object and discarding inherited properties. Deep merging is provided by libraries such as lodash.merge or by recursive implementations that merge each nested key rather than replacing it. Arrays are often treated differently: some merges replace arrays, others concatenate or merge elements by index.

Typical use cases include combining default options with user-provided options, assembling configuration from multiple sources, or merging API responses with local overrides. When designing a merge strategy, consider immutability, performance, and how to handle nulls or undefined values.

Potential pitfalls include unintended mutation of inputs, loss of nested defaults, and prototype pollution risks if keys are not sanitized. In JavaScript, avoid merging properties into Function.prototype or Object.prototype, and prefer merging into a fresh object or a dedicated configuration object. Tests should cover edge cases such as deeply nested structures and arrays.