Function isPlainObject

isPlainObject<T extends object>(value: T): value is T
Type Parameters
- T extends object
Parameters
- value: T
Returns value is T
isPlainObject(value: unknown): value is Record<PropertyKey, unknown>
Determines whether a value is a plain object.

A plain object is one whose prototype is either:
- Object.prototype (created via {} or new Object())
- null (created via Object.create(null))
This excludes arrays, functions, class instances, DOM objects, and any object with a custom prototype. In other words, this function detects JSON-like dictionary objects rather than structural or callable object types.

Type Behavior:
- If the input already has a known object type T, that type is preserved after narrowing.
- If the input is unknown or untyped the result narrows to Record<PropertyKey, unknown> allowing safe keyed access.
Useful when validating configuration objects, cloning or merging data, performing deep equality, or working with structured JSON where non-plain / prototype values would be considered invalid.
Parameters
- value: unknown
  Any value to evaluate.
Returns value is Record<PropertyKey, unknown>
True if the value is a plain object with no special prototype.
Example
```
const a = { x: 1 };
isPlainObject(a);   // true

class Foo {}
isPlainObject(new Foo()); // false
```
Example
```
let data: unknown = getValue();
if (isPlainObject(data)) {
  data.foo;         // ok — key is `unknown`, but structure is guaranteed.
}
```