Does passing an object via props to child react component, clone the original object or pass by reference?

In general, it depends on the data-type of said prop. Primitives, such as integers or Strings are passed down by their value, while Object data-types such as arrays are passed down by their reference.

So yes, Objects specifically are passed down by reference.

Demo:

In this demo we have two components, one parent and one child. The child takes two props, one that is the integer 1 and the other is an object with a: "foo".

After a short while, from the parent we change the integers value from 1 to 2 and the object from a: "foo" to a: "bar". After that, we log the values of these props from the child component.
Note: This is a stateless component, so we aren't using any kind of state. We are changing the values directly, so React won't ever "know" of this change! _{(ssh, don't tell React!)}

The output is still 1 for the integer (didn't change - passed by value), but a: "bar" for the object (changed! - passed by reference).

const Parent = () => { 
  let myInt = 1;
  let myObj = {a: "foo"};
  setTimeout(function(){
    myInt = 2;
    myObj.a = "bar";
  }, 500);
 
  return <Child myInt={myInt} myObj={myObj} />;
}

const Child = ({myInt, myObj}) => {
  setTimeout(function(){
    console.log(myInt);
    console.log(myObj);
  }, 1000);
  
  return (
    <div>
      <p>{myInt}</p>
      <p>{JSON.stringify(myObj)}</p>
    </div>
  );
}
 
ReactDOM.render(<Parent />, document.getElementById("app"));

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id="app"></div>

Props reference to objects, in fact you can pass methods with context. Mutating props is a very bad pratice discouraged by almost every linter, when you want to mutate data coming from props is better to deep clone prop and work on cloned data.

Only a reference to the object is passed. Indeed, the transformation is pretty straightforward between jsx/tsx and plain old js/ts. This babel reference is a good example of this.

This means that any sort of object can potentially be sent - numbers, booleans, arrays, functions, plain old objects, DOM elements etc. It also means that if the object is mutable, it could be modified. But it should not.