Function assertPlainObject

assertPlainObject<T>(value: T, errorMsg?: string): asserts value is T & object
Asserts that a value is a plain object, not null, and not an array.

Unlike isPlainObject, this function does not narrow the value to a generic indexable structure. Instead, it preserves the existing static type of the variable. This makes it ideal for validating option objects or interface-based inputs where all properties may be optional.

Use this function when:
```
  - You expect a value to be a plain object at runtime, **and**
  - You want to keep its compile-time type intact after validation.
```
Type Parameters
- T
Parameters
- value: T
  The value to validate.
- OptionalerrorMsg: string
  Optional message used for the thrown TypeError.
Returns asserts value is T & object
Example
```
interface Options { flag?: boolean; value?: number; }

function run(opts: Options = {}) {
  assertPlainObject(opts, `'opts' is not a plain object.`); // `opts` remains `Options`, not widened or reduced.
  opts.value;                                               // Fully typed access remains available.
}
```
Throws
if the value is null, non-object, or an array.