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Pre-loading text in textarea for user to edit and submit shows text briefly then blanks out. Why?

I'm writing an edit function (plain javascript & HTML / Chrome / Windows 10).
The data is in localStorage as a series of records, just 2 records in the toy code mentioned below.
I want the user to specify the number of the record to edit, then the code should pre-fill the textarea field with the retrieved content of that record. I want to allow the user to make changes and then press a Store button to store it back in localStorage.
My problem is that when I prefill the input field, I see the record content briefly and then the input field clears. I've tried .value and .defaultValue
editField.value = localStorage.getItem('jnl' + locStoreNo).replace(/(.*?) `\d*?`/, "$1");
and
editField.defaultValue = localStorage.getItem('jnl' + locStoreNo).replace(/(.*?) `\d*?`/, "$1");
the result is the same. (The regex is to hide a sequence number)
The code is in a JSFiddle here: https://jsfiddle.net/roygrubb/zxedbfqr/2/
That performs more or less the same - it shows the value briefly - but then does something different: It goes to a 404. I don't understand this either ¯_(ツ)_/¯
What I'm trying to do seems so basic, that I think I must be missing something blindingly obvious.
What have I missed? Thanks!

Whenever you've got a <form> that you want to handle through JavaScript, you have to ensure that the default form submission action does not happen. If the <form> does not have an "action" attribute, the default is to reload the current page.
By default, a <button> element will be assumed to have "submit" as its type. To prevent form submission, therefore, the simplest thing to do is make the button have "button" as its type.
That may not be all you need to do, depending on the details of the form. It may be necessary (or simply a good defensive move) to have a handler for the "submit" event on the form to prevent the default action.

Fill an input field that is not of type text and that triggers events in CasperJS

I have to do automated tests on a website and I want to use CasperJS to learn. For proprietary reasons I can not give too much code.
Here is the example of the input that I am trying to fill:
<input data-bind="value: firstname, valueUpdate: ['blur'], css: {valid:(firstname.isValid() )} " title="" class="valid" aria-required="true" id="firstname" name="firstname">
As you can see, this input is not of type text and has no value attribute. Therefore, I can not use the casper.fill() method. Furthermore, if I enter the web page scope using evaluate() and change the input value using document.querySelector, the change will not be permanent as of the events attached to the text change on the input will not be triggered.
Here is my code:
this.waitForSelector('#memberTitle', function then(){
var testname = 'thisIsNotPermanent';
this.evaluate(function(testname){
document.querySelector('#firstname').value = testname;
}, testname);
});
If I capture the screen right after, I will see my text written in the input box. However, if I wait 500ms and take another capture, the text is gone as, I suppose, the events are triggered or just cleaned because it actually failed to trigger correctly.
The events attached to the input are of Blur, Change and Keypress.
Using CasperJS, how could I go to the lowest level possible to mimic a user using his keyboard and fully use the website's functionalities already in place?
The whole point of those tests are to work with what is in place. The idea is to not have to manually go through the JavaScript of the web site.

That's exactly what the casper.sendKeys(selector, keys) function is for which will send native keypresses and (hopefully) trigger the events on that text element:
this.waitForSelector('#memberTitle', function then(){
var testname = 'thisIsNotPermanent';
this.sendKeys('#firstname', testname);
}).wait(20, function(){
this.capture('screenshot.png');
});
<input> elements without a type attribute default to Text type.

This answer is here to complete the question from another angle. As Artjom B. mentionned, the correct way to fill an input and to trigger its events is by using the sendKeys() function. However, if you ever have a case, like mine, where the events will not trigger or will take a certain amount of time, know that you can trigger those manually.
If you use the firefox inspector tool, you will see that your input or tag will have an event attached to it marked as ev. If you select it, you will have a breakdown of all the events, in order, that are triggered.
You can see that the jQuery click() event will be called. In casperjs, from the evaluate scope you can now do this :
this.evaluate(function(){
$(".discard-answer").click();
})
From there, you can chain jQuery events, like in my case where I had to .blur().change().click();
It is important to know if the event is jQuery or not.
Hope this helps.

Simple .on('keydown') jQuery event not responding until next key is pressed

I'm making a page for a friend and I have a hidden text field and when the user types the text is transposed into a div so that it looks like they're typing on the screen rather than in an input field.
Here is a link to the page: http://merkd.com/godis.php
Here is the function that I use to respond to the key strokes:
$('#hiddenInput').keydown(function() {
var input = $('#hiddenInput').val();
var html = '<div style="float: left;">'+input+'</div><div id="cursor">|</div>';
$('#typingArea').html(html);
});
The text-field is visible right now so that you can see the problem. When text is entered or deleted, it doesn't respond until the next keypress. So if I type a single letter, nothing shows up until I type the next letter.
I looked at the jQuery .on() documentation but I couldn't find anything on this. Any help is much appreciated.
P.S. I know it should be in a separate question, but is there an easy way to make a text-field always in focus? I want to make it so that no matter where the user clicks or whatever, if they type, the text will still show up.

Use .keyup() event because when you first press (keydown), the letter is never typed so the var html is getting previous value. For second part you can bind keypress event in document to focus your input field.

How can I keep the focus on an input field if the input is incorrect

Given the following code:
sPosSelect="#fpJpos input[name=posnum" + iiPos + "]";
if (fIn>fPosMaxWtUse[iiPos]) {
alert(sprintf('%.0f is %.0f more than the position max of %.0f.',fIn,fIn-fPosMaxWtUse[iiPos],fPosMaxWtUse[iiPos]));
$(sPosSelect).val('');
$(sPosSelect).focus();
return;
}
It works in that I get the alert, and the field is blanked. However, the focus then moves on to the next field when what I want is for it to stay on the field just blanked so the user can try again.
All suggestions are welcome, including anything I'm doing that could be done in a better way.
Terry

I assume your code is within an event attached to the field in question, presumably on blur?
If this is the case, you should simply use return false at the end of the function. This will tell the browser to ignore it's default behaviour, which in this case would be moving to the next field.
What your code is doing at the moment is setting focus in the field, and then returning control to the browser, which assumes everything went okay, and happily moves on to the next field.
Also, if this code is within an event attached to the field, you should really be using $(this) in place of repeating the selector $(sPosSelect).

Dynamically growing an array of text inputs (HTML/JavaScript)

I'm creating a data entry app for some in-house stuff.
My team needs to enter info about "items" which can have many "categories" and vice versa.
I need a quick way to let them enter an arbitrary amount of categories.
Here's my idea:
On the item entry page, I'll have it so that initially there's one text input for "categories" and if it's tabbed out of while it's empty, the input field is deleted (unless it's the only one) and focus skips to the next field. If it's not empty when it's tabbed out of and if it's the last input field in the array, then an additional "category" text input will be added and focused.
This way people can enter an arbitrary amount of categories really quickly, without taking their hands off the keyboard, just by typing and hitting tab. Then hitting tab twice to denote the end of the list.
First of all, what do you think of this interface? Is there a better way to do it?
Second of all, is there a jQuery (or something) plugin to do this? I've searched but can't find one. I searched scriptaculous/prototype and mootools too, with no luck.
I would obviously rather use something tried and tested than roll my own.
Any and all advice appreciated

First I'll try to address the problems commented on nickf solution.
To set the focus on the newly created input $copy.find(":text").focus(); will not work. The jQuery focus method only triggers the event, but does not call the underlying focus method.
You can set the focus with setTimeout(function(){$copy.find(":text").get(0).focus()}, 10); but:
setTimeout is needed in firefox or strange things will happen with the blinking cursor.
IE7 needs another input to focus when tabbing. I haven't found the way to set the focus on an input if the focus goes to the address bar. I suppose this will not be a problem because you will need at least a submit button.
To control shift-tab I've been trying to track the focused element, in order to skip the blurHandler when the focused element is a previous input, but the resulting code is really ugly so I'll post this and look for a better solution.
And last, you're asking what we think of this UI, and I think that a comma separated list of categories is easier to code an to fill in. :-)

it's actually not too difficult to implement that, even with vanilla JS (ie: no jQuery, prototype, etc), but everything is easier with jQuery, so I'll have a go at it using that:
Assuming a structure like this:
<form id="myForm">
<div class="inputRow">
<input type="text" name="myInput[]" />
</div>
<div class="inputRow">
<input type="text" name="myInput[]" />
</div>
...
</form>
Here's the JS
$('#myForm :text').blur(onBlurHandler);
function onBlurHandler() {
$row = $(this).parent();
if ($row
.nextAll(":has(:text)") // all following divs with a text element
.length == 0 // but there aren't any, we're on the last one
) {
if ($.trim($row.find(":text").val())) { // the text box isn't empty
$copy = $row.clone(true);
$copy
.find(":text") // get the new text box,
.val('') // remove any text in it
.blur(onBlurHandler) // and add the event handler (is this necessary?)
;
$copy.insertAfter($row);
} else if ($row.prev(':has(:text)').length) { // the text box is empty, and this one isn't the first row
$row.remove(); // get rid of the row.
}
}
}
Response to comments:
thanks for the answer! i've tried it but it doesn't seem to work as intended. i'm on mac firefox. if i tab off the last field, it adds the new one but focuses the address bar. i tried adding: $copy.find(":text").focus(); after the insertAfter line, but it doesn't change anything. any ideas?
also if i shift-tab the blurhandler doesn't know i'm going in the opposite direction. is there any way around that?
Hmm, I hadn't thought about that. What you could try doing is to put an element after all your text fields which can take focus (like a textbox which is rendered off-screen, eg: margin-left: -10000px). Add an onfocus handler onto that to see if the last row is empty, and if it is, then it would have been added just then by the onBlurHandler function, so pass the focus back to the last row. If the last row isn't empty, then pass the focus onto the next element (your submit button, probably). If there are issues with the last row not existing in the DOM yet, then put the above into a timeout.
(If this actually works) this should let your users tab backwards and forwards without hassle.