Is the (pseudo?) code there for state mutation representative of a widely used pattern? It looks like using a pattern from a functional programming book, but applied to mutable data. What I mean is this snippet:

    function handleAddTodo(state) {
       if (!newTodoField) {
         state.error = 'Empty field!'
       }

       state.todos.push(state.newTodoField)
       state.newTodoField = nil

       return state;
    }

This looks like a function newState = fun(oldState) but there is one huge problem: the new state is the same data as the old state? In a Rust-like type system this might be OK because you couldn't accidentally keep using the original state after sending it to this method, and in an immutable scenario there would be no problem because you created a new state instead of mutating. But this looks exactly like mutating the argument sent to a function. So out of curiosity, do any modern UI frameworks in JS (this looks like that type of code) do this, or was it just an unfortunate example?