The object is still passed by value, however the object itself contains references, which may be mutated. If objects were pass by reference then you’d end up with {a: 2} at the end:

    let a = {a: 1}
    
    function foo(a) {
      a = {a: 2}
    }
    
    foo(a)
    a //=> {a: 1}

Don’t confuse passing a mutable entity by value with pass by reference.