The behavior property of the ScrollToOptions dictionary specifies whether the scrolling should animate smoothly, or happen instantly in a single jump.
This is actually defined on the ScrollOptions dictionary, which is implemented by ScrollToOptions.
behavior: ScrollBehavior
An enum, the value of which can be one of the following:
smooth: The scrolling animates smoothly.auto: The scrolling happens in a single jump.In our scrolltooptions example (see it live) we include a form that allows the user to enter three values — two numbers representing the left and top properties (i.e. the positions to scroll to along the X and Y axes), and a checkbox indicating whether they want smooth scrolling enabled or not.
When the form is submitted, an event handler is run that puts the entered values into a ScrollToOptions dictionary, and then invokes the Window.ScrollTo() method, passing the dictionary as a parameter:
form.addEventListener('submit', (e) => {
  e.preventDefault();
  scrollOptions = {
    left: leftInput.value,
    top: topInput.value,
    behavior: scrollInput.checked ? 'smooth' : 'auto'
  }
  window.scrollTo(scrollOptions);
}); | Specification | Status | Comment | 
|---|---|---|
| CSS Object Model (CSSOM) View Module The definition of 'behavior' in that specification.  |  Working Draft | 
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    https://developer.mozilla.org/en-US/docs/Web/API/ScrollToOptions/behavior