Unresolved variable valueAsNumber

Ok guys here is the the code (which is working on all browsers flawlessly) :

[code]

<div>
    <button onclick="htmlHandlers.changeTodo()">Change Todo</button>
    <input id="changeTodoPositionInput" type="number">
    <input id="changeTodoTextInput" type="text">
</div>

[/code]

[code]

const htmlHandlers = {
    displayTodos: function() {
        todoList.displayTodos();
    },
    toggleAll: function() {
        todoList.toggleAll();
    },
    addTodo: function() {
        const addTodoTextInput = document.getElementById('addTodoTextInput');
        todoList.addTodo(addTodoTextInput.value);
        addTodoTextInput.value = '';
    },
    changeTodo: function() {
        const changeTodoPositionInput = document.getElementById('changeTodoPositionInput');
        const changeTodoTextInput = document.getElementById('changeTodoTextInput');
        todoList.changeTodo(changeTodoPositionInput.valueAsNumber, changeTodoTextInput.value);

        changeTodoPositionInput.value = '';
        changeTodoTextInput.value = '';
    }

};

[/code]

The question is why I am getting the error 'Unresolved variable valueAsNumber' ?