Function isPlainObject

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<string, 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.

const a = { x: 1 };
isPlainObject(a);   // true

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