working with the HTML textbox onchange event with JavaScript

I saw this example
https://www.w3schools.com/tags/tryit.asp?filename=tryhtml5_ev_onchange2

When you run it, it looks like this. So the textbox has the value Hello.

p1.PNG
Now I type the word World in the textbox and click outside the textbox which fires the onchange event like this.

p2.PNG

Is there a way to store the existing value of the textbox in a hidden field and also store the new text i typed in the textbox in a hidden field also, but separately.

So when I run the above example and then type World.

I want to store the existing value which is "Hello" in a hidden field called HiddenField1
and the new text that I typed in the textbox which is "World" in a hidden field called HiddenField2

What's the best way to do that?
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maqskywalkerAsked:
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Ryan ChongBusiness Systems Analyst , ex-Senior Application EngineerCommented:
aside of onchange event, try look for onfocus event to store the previous value. similarly, look for onblur event in case you want to store the new value somewhere else (if there's necessary)
Zakaria AcharkiAnalyst DeveloperCommented:
The best way for this case is to avoid the use of inline-events like `onchange / on..` (attaching the event from the HTML code is the old-fashioned way), you need to attach your event in the JS side by using `addEventListener()` method, and I would suggest the use of blur (the opposite of focus) that will fire once you leave the input (lose the focus).

For the first hidden field you could init it by default using the same default value of the text input, and for the second hidden field you must update the value every time the blur is triggered like :

document.querySelector('[name="txt"]').addEventListener('blur', function() {
    document.querySelector('[name="HiddenField2"]').value = this.value;
});

Open in new window


Here is a working sample :

http://jsfiddle.net/z_acharki/qL5zn7wc/
maqskywalkerAuthor Commented:
Zakaria,
I looked at your example that's close. The value stored in HiddenField1 is ok.

But I noticed the value being stored in HiddenField2 is the full string. I only want to store whatever I type in the texbox.

The texbox already contains "Hello" on page load.
So then if I type "testing 123" in the texbox after the word Hello , the only value I want saved in HiddenField2 is what I typed.
So HiddenField2 would contain the value "testing 123" not "Hello testing 123" like the fiddle is currently doing.

How do I revise it to do that?
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Zakaria AcharkiAnalyst DeveloperCommented:
That could be achieved by excluding the value of the hidden field 1 from the second hidden field like :

var main_field = document.querySelector('[name="txt"]');
var hidden_field_1 = document.querySelector('[name="HiddenField1"]');
var hidden_field_2 = document.querySelector('[name="HiddenField2"]');

main_field.addEventListener('blur', function() {
  hidden_field_2.value = this.value.replace(hidden_field_1.value, "");

  console.log("The Hidden field 1 value is : " + hidden_field_1.value);
  console.log("The Hidden field 2 value is : " + hidden_field_2.value);
});

Open in new window


Here is the updated fiddle :

http://jsfiddle.net/z_acharki/pbefgn2r/1/

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maqskywalkerAuthor Commented:
thanks
Zakaria AcharkiAnalyst DeveloperCommented:
Glad I could help.
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