[feature request] add new isPlainObject predicate function #32

The isPlainObject predicate function is another common convenience function in the JS Object domain.

• It is quite frequently used and would be very helpful to have.
• It is distinct from the very similar isObject predicate function from @openinf/util-types
• It is distinct from the isObjectLike predicate function, which is often kept private/internal

const isPlainObject = ( obj ) => {
	return (
		typeof obj === 'object' &&
		obj !== null &&
		obj.constructor === Object &&
		Object.prototype.toString.call( obj ) === '[object Object]'
	);
};

Aside

hopefully not a misnomer due to the existence of plain old Java objects (POJOs), which are also a thing in JS

Open question

would a more rigorous check be necessary to differentiate btwx the two or is it fine?