Form properties and methods

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

Working with forms can be much more convenient if we know them.

Navigation: form and elements

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

That’s a named collection: we can use both the name and the number 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, that’s often the case with radio buttons.

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].value); // 10, the first input value
</script>

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

Fieldsets as “subforms”

A form may have one or many <fieldset> elements inside it. They also support the elements property.

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 both from the form and from the fieldset
    alert(fieldset.elements.login == form.elements.login); // true
  </script>
</body>
Shorter notation: form.name

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

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

  // the direct access now can use both names: the new one and the old one
  alert(form.username == form.login); // true
</script>

That’s usually not a problem, 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, pay attention to their specific features.

input and textarea

Normally, we can access the value as input.value or input.checked for checkboxes.

Like this:

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

input.checked = true; // for a checkbox or radio button
Use textarea.value, not textarea.innerHTML

Please note that we should never use textarea.innerHTML: 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:

  1. select.options – the collection of <option> elements,
  2. select.value – the value of the chosen option,
  3. select.selectedIndex – the number of the selected option.

So we have three ways to set the value of a <select>:

  1. Find the needed <option> and set option.selected to true.
  2. Set select.value to the value.
  3. Set select.selectedIndex to the number of the option.

The first way is the most obvious, but (2) and (3) are usually more convenient.

Here is an example:

<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';
</script>

Unlike most other controls, <select multiple> allows multiple choice. In that case we need to walk over select.options to get all selected values.

Like this:

<select id="select" multiple>
  <option value="blues" selected>Blues</option>
  <option value="rock" selected>Rock</option>
  <option value="classic">Classic</option>
</select>

<script>
  // get all selected values from multi-select
  let selected = Array.from(select.options)
    .filter(option => option.selected)
    .map(option => option.value);

  alert(selected); // blues,rock
</script>

The full specification of the <select> element is available at https://html.spec.whatwg.org/multipage/forms.html#the-select-element.

new Option

In the specification of the option element there’s a nice short syntax to create <option> elements:

option = new Option(text, value, defaultSelected, selected);

Parameters:

  • text – the text inside the option,
  • value – the option value,
  • defaultSelected – if true, then selected attribute is created,
  • selected – if true, then the option is selected.

For instance:

let option = new Option("Text", "value");
// creates <option value="value">Text</option>

The same element selected:

let option = new Option("Text", "value", true, true);
Additional properties of <option>

Option elements have additional properties:

selected
Is the option selected.
index
The number of the option among the others in its <select>.
text
Text content of the option (seen by what the visitor).

Summary

Form navigation:

document.forms
A form is available as document.forms[name/index].
form.elements
Form elements are available as form.elements[name/index], or can use just form[name/index]. The elements property also works for <fieldset>.
element.form
Elements reference their form in the form property.

Value is available as input.value, textarea.value, select.value etc, or input.checked for checkboxes and radio buttons.

For <select> we can also get the value by the index select.selectedIndex or through the options collection select.options. The full specification of this and other elements is at https://html.spec.whatwg.org/multipage/forms.html.

These are the basics to start working with forms. In the next chapter we’ll cover focus and blur events that may occur on any element, but are mostly handled on forms.

Tasks

importance: 5

There’s a <select>:

<select id="genres">
  <option value="rock">Rock</option>
  <option value="blues" selected>Blues</option>
</select>

Use JavaScript to:

  1. Show the value and the text of the selected option.
  2. Add an option: <option value="classic">Classic</option>.
  3. Make it selected.

The solution, step by step:

<select id="genres">
  <option value="rock">Rock</option>
  <option value="blues" selected>Blues</option>
</select>

<script>
  // 1)
  let selectedOption = genres.options[select.selectedIndex];
  alert( selectedOption.value );

  // 2)
  let newOption = new Option("classic", "Classic");
  select.append(newOption);

  // 3)
  newOption.selected = true;
</script>
Tutorial map

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