July 3, 2019

Forms: event and method submit

The submit event triggers when the form is submitted, it is usually used to validate the form before sending it to the server or to abort the submission and process it in JavaScript.

The method form.submit() allows to initiate form sending from JavaScript. We can use it to dynamically create and send our own forms to server.

Let’s see more details of them.

Event: submit

There are two main ways to submit a form:

The first – to click <input type="submit"> or <input type="image">.
The second – press Enter on an input field.

Both actions lead to submit event on the form. The handler can check the data, and if there are errors, show them and call event.preventDefault(), then the form won’t be sent to the server.

In the form below:

Go into the text field and press Enter.
Click <input type="submit">.

Both actions show alert and the form is not sent anywhere due to return false:

          <form onsubmit="alert('submit!');return false">
  First: Enter in the input field <input type="text" value="text"><br>
  Second: Click "submit": <input type="submit" value="Submit">
</form>

When a form is sent using Enter on an input field, a click event triggers on the <input type="submit">.

That’s rather funny, because there was no click at all.

Here’s the demo:

          <form onsubmit="return false">
 <input type="text" size="30" value="Focus here and press enter">
 <input type="submit" value="Submit" onclick="alert('click')">
</form>

Method: submit

To submit a form to the server manually, we can call form.submit().

Then the submit event is not generated. It is assumed that if the programmer calls form.submit(), then the script already did all related processing.

Sometimes that’s used to manually create and send a form, like this:

let form = document.createElement('form');
form.action = 'https://google.com/search';
form.method = 'GET';

form.innerHTML = '<input name="q" value="test">';

// the form must be in the document to submit it
document.body.append(form);

form.submit();

Tasks

Modal form

Create a function showPrompt(html, callback) that shows a form with the message html, an input field and buttons OK/CANCEL.

A user should type something into a text field and press Enter or the OK button, then callback(value) is called with the value they entered.
Otherwise if the user presses Esc or CANCEL, then callback(null) is called.

In both cases that ends the input process and removes the form.

Requirements:

The form should be in the center of the window.
The form is modal. In other words, no interaction with the rest of the page is possible until the user closes it.
When the form is shown, the focus should be inside the <input> for the user.
Keys Tab/Shift+Tab should shift the focus between form fields, don’t allow it to leave for other page elements.

Usage example:

          showPrompt("Enter something<br>...smart :)", function(value) {
  alert(value);
});
        

A demo in the iframe:

P.S. The source document has HTML/CSS for the form with fixed positioning, but it’s up to you to make it modal.

Open a sandbox for the task.

A modal window can be implemented using a half-transparent <div id="cover-div"> that covers the whole window, like this:

          #cover-div {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 9000;
  width: 100%;
  height: 100%;
  background-color: gray;
  opacity: 0.3;
}
        

Because the <div> covers everything, it gets all clicks, not the page below it.

Also we can prevent page scroll by setting body.style.overflowY='hidden'.

The form should be not in the <div>, but next to it, because we don’t want it to have opacity.

Open the solution in a sandbox.

Tutorial map

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