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.

{{page("/en-US/docs/Web/API/HTMLFormElement", "Elements that are considered form controls")}}

numControls = form.length;

numControls is the number of form controls within the <form>. This is the same as the number of the elements in the HTMLFormControlsCollection returned by the elements property.

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