Form properties and methods

Forms and control elements, such as <input> have a lot of special properties and events.

Working with forms will be much more convenient when we learn them.

Navigation: form and elements

Document forms are members of the special collection document.forms.

That’s a so-called “named collection”: it’s both named and ordered. We can use both the name or the number in the document to get the form.

          document.forms.my; // the form with name="my"
document.forms[0]; // the first form in the document
        

When we have a form, then any element is available in the named collection form.elements.

For instance:

<form name="my">
  <input name="one" value="1">
  <input name="two" value="2">
</form>

<script>
  // get the form
  let form = document.forms.my; // <form name="my"> element

  // get the element
  let elem = form.elements.one; // <input name="one"> element

  alert(elem.value); // 1
</script>

There may be multiple elements with the same name. This is typical with radio buttons and checkboxes.

In that case, form.elements[name] is a collection. For instance:

<form>
  <input type="radio" name="age" value="10">
  <input type="radio" name="age" value="20">
</form>

<script>
let form = document.forms[0];

let ageElems = form.elements.age;

alert(ageElems[0]); // [object HTMLInputElement]
</script>

These navigation properties do not depend on the tag structure. All control elements, no matter how deep they are in the form, are available in form.elements.

A form may have one or many <fieldset> elements inside it. They also have elements property that lists form controls inside them.

For instance:

<body>
  <form id="form">
    <fieldset name="userFields">
      <legend>info</legend>
      <input name="login" type="text">
    </fieldset>
  </form>

  <script>
    alert(form.elements.login); // <input name="login">

    let fieldset = form.elements.userFields;
    alert(fieldset); // HTMLFieldSetElement

    // we can get the input by name both from the form and from the fieldset
    alert(fieldset.elements.login == form.elements.login); // true
  </script>
</body>

There’s a shorter notation: we can access the element as form[index/name].

In other words, instead of form.elements.login we can write form.login.

That also works, but there’s a minor issue: if we access an element, and then change its name, then it is still available under the old name (as well as under the new one).

That’s easy to see in an example:

<form id="form">
  <input name="login">
</form>

<script>
  alert(form.elements.login == form.login); // true, the same <input>

  form.login.name = "username"; // change the name of the input

  // form.elements updated the name:
  alert(form.elements.login); // undefined
  alert(form.elements.username); // input

  // form allows both names: the new one and the old one
  alert(form.username == form.login); // true
</script>

That’s usually not a problem, however, because we rarely change names of form elements.

Backreference: element.form

For any element, the form is available as element.form. So a form references all elements, and elements reference the form.

Here’s the picture:

For instance:

<form id="form">
  <input type="text" name="login">
</form>

<script>
  // form -> element
  let login = form.login;

  // element -> form
  alert(login.form); // HTMLFormElement
</script>

Form elements

Let’s talk about form controls.

input and textarea

We can access their value as input.value (string) or input.checked (boolean) for checkboxes.

Like this:

          input.value = "New value";
textarea.value = "New text";

input.checked = true; // for a checkbox or radio button
        

Please note that even though <textarea>...</textarea> holds its value as nested HTML, we should never use textarea.innerHTML to access it.

It stores only the HTML that was initially on the page, not the current value.

select and option

A <select> element has 3 important properties:

select.options – the collection of <option> subelements,
select.value – the value of the currently selected <option>,
select.selectedIndex – the number of the currently selected <option>.

They provide three different ways of setting a value for a <select>:

Find the corresponding <option> element (e.g. among select.options) and set its option.selected to true.
If we know a new value: set select.value to the new value.
If we know the new option number: set select.selectedIndex to that number.

Here is an example of all three methods:

<select id="select">
  <option value="apple">Apple</option>
  <option value="pear">Pear</option>
  <option value="banana">Banana</option>
</select>

<script>
  // all three lines do the same thing
  select.options[2].selected = true;
  select.selectedIndex = 2;
  select.value = 'banana';
  // please note: options start from zero, so index 2 means the 3rd option.
</script>

Unlike most other controls, <select> allows to select multiple options at once if it has multiple