Function deepMerge

deepMerge<T extends Record<string, unknown>>(
target: T,
...sources: (Record<string, unknown> | null | undefined)[],
): T
Recursively merge properties from source objects into the target object, mutating it.

Nested plain objects are merged recursively, arrays are merged by index (e.g., [1, 2] + [3] → [3, 2]), and class instances (Date, RegExp, custom classes) are assigned by reference. Circular references are detected via ancestor-chain tracking and safely skipped, while shared (non-circular) object references are merged correctly. Prototype pollution attempts (__proto__, constructor) are also skipped.
Type Parameters
- T extends Record<string, unknown>
Parameters
- target: T
  The target object to merge into (mutated)
- ...sources: (Record<string, unknown> | null | undefined)[]
  One or more source objects to merge from
Returns T
Example
```
import { deepMerge } from '@aws-lambda-powertools/commons';

const target = { a: 1, nested: { x: 1 } };
const source = { b: 2, nested: { y: 2 } };
const result = deepMerge(target, source);
// result === target === { a: 1, b: 2, nested: { x: 1, y: 2 } }
```
- Defined in deepMerge.ts:216