HTMLFormElement.length

The HTMLFormElement.length read-only property returns the number of controls in the <form> element.

You can access the list of the form's controls using the elements property.

This includes both elements that are descendants of the <form> element as well as elements that are made members of the form using their form property.

Elements that are considered for this property are: <button>, <fieldset>, <input> (with the exception that any whose type is "image" are omitted for historical reasons), <object>, <output>, <select>, and <textarea>.

if (document.getElementById('form1').length > 1) {
  // more than one form control here
}