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Getting element value in javascript returns null [duplicate]

I'm just wondering what the difference is between the two. I have noticed the two methods give different results at times.

The difference is that element.value is real time and if a user changes let's say, a textbox input, it will reflect that, and show you the new value.
While getAttribute('value') will still show the original value="whateverWasHere" value.
jsFiddle DEMO

.value does not map to any attribute.
.defaultValue maps to the "value" attribute. So when you say elem.getAttribute("value") that's the same as elem.defaultValue.
Additionally, .defaultValue reflects .value when the input is untouched (dirty value flag is false). After the input's value is changed by user interaction, this mapping stops. While the input is untouched, you can change .defaultValue (and thus .setAttribute("value")) and see it change .value as well. Not that this is practically useful but interesting piece of trivia nevertheless.

confused with input element with/without value attribute

I'm not new to HTML but I always get confused with input elements, below is some html:
<input id="firstTest" />
<input id="secondTest" value="Hello"/>
so for the firstTest input, I typed some text like "Hi",then do:
let firstInput = document.getElementById('firstTest');
console.log(firstInput.value) //produces "Hi"
but for the secondTest input, I did the same thing but the value is always "Hello" and I can't type anything into the input field.
So my questions are:
Q1-why for the firstInput, I didn't specify a value attribute but the value can change depending on what I typed?
Q2-when I type sth into the field, what actually happened? How browser display the thing I type?
does input object in DOM get its value property updated automatically then the browser displays the latest value on screen?
Q3-if input object in DOM get its value property updated automatically, why on the secondInput that has a value attribute couldn't get its value property updated automatically?

Q1-why for the firstInput, I didn't specify a value attribute but the value can change depending on what I typed?
The value HTML attribute serves only for the purpose of defining an initial value. input elements have a Javascript API, and in this, there is a value property (which you are using in your console.log). This value property constantly reflects whatever is in the input. If you want to know what is in the value attribute, there's two ways to find out:
el.getAttribute('value');
or, using the aforementioned API
el.defaultValue;
Q2-when I type sth into the field, what actually happened? How browser display the thing I type? does input object in DOM get its value property updated automatically then the browser displays the latest value on screen?
That's a matter of how the browser handles it internally. This is an implentation detail which is never relevant for the web developer. It is laid out in the specification for that element.
Q3-if input object in DOM get its value property updated automatically, why on the secondInput that has a value attribute couldn't get its value property updated automatically?
Given the code you show here, that cannot be the case. The only ways to disable typing in an input is if the input has a disabled or readonlyattribute, or if someone has added a keydown listener that calls event.preventDefault().
If this does not fully answer your question, please leave a comment stating what is remaining unclear or unanswered.

Value - property and attribute

I tried to update input element's value using .val() and it turned out that it doesn't affect the value="" attribute. After reading some topics here on stackoverflow, it turned out that these are not the same.
I started changing them both to be sure I do not do any mistake there:
$("#elementid").val(variableNumber).attr('value', variableNumber);
Now, lets say that I am sending the value of my input to validate it in php.
Which of these will be sent ff I make them different?
$("elementid").val(variable1);
$("elementid").attr('value', variable2);
Is there any rule on this? or any factor that makes one of these being sent as the actual "value" ?

Which of these will be sent ff I make them different?
The input's current value will be sent. The current value is reflected by the the value property. The value attribute represents the default value of the input, not its current value (and is reflected as the defaultValue property on the input). The default value is used to initialize the value when the input is created, and to reset it if you use the reset method of a form it's in.
Unless you want to change the default value, there's no need to set the value attribute, just the property. The property is what val changes, what changes when the user acts on the input, and what gets sent when the form is submitted.

When you are working with <input type='text'> I think you should use attr('value', variableNumber); instead of val(value).
val(value) is useful when working on a jQuery object containing elements like <input type="checkbox">, <input type="radio">, and <option>s inside of a <select>. More infomation here.

html input displays different text to value that it posts

I am feeling really stupid and not sure if doing something really dumb what... so excuse me in advance.
I have the following HTML:
<input type="text" value="0" name="pcscubes" id="pcscubes">
I have the below jquery syntax:
var cubes=123.2;
$("#pcscubes1").val(cubes);
var test=$("#pcscubes1").val();
alert(test);
When the jquery executes, the input box will display 123.2 and the alert will be 123.2.
if you inspect the html object though, the value is still 0?
so the display of the input is different from what the real value is. so when I submit this form it posts 0 rather than 123.2
Now I have built a whole web app with this syntax and works perfectly. today it just stops working? I have restarted browser, checked old code that was working, logged off and on and still it does the same.
web app obviously has jquery loaded and jquery ui on this form.
using codeigniter?
any ideas as I am banging my head over something so stupid....
Out of interest I created the below fiddle and it is doing the same?
http://jsfiddle.net/49nqU/
why is the value 0 when it is showing 123.2?
Thanks as always,

This is a case of attributes vs. properties.
What you are seeing in the inspector is the value attribute, which does not update when you programmatically update a field's value property.
Pay no attention to what the DOM inspector says; it'll only show the value as it was when the page loaded (unless you explicitly change the field's value attribute rather than, as jQuery's val() does, its value property.)
Further reading: http://jquery-howto.blogspot.co.uk/2011/06/html-difference-between-attribute-and.html
Some attributes are closely tied to their property counterparts. That is to say, updating the property also updates the attribute. This is the case, for example, with the class attribute and its className property counterpart. value, however, is different; updating the property does not update the attribute (but, just to confuse things further, updating the attribute does update the property!)

val() is changing the value property, not attribute. The value attribute is initialized at the loading of the page. Once you change text field the value property and attribute have differnet values.
But you can use plain JS for changing the value attribute also.
Try:
var cubes=123.2;
document.getElementById('qty').setAttribute('value',cubes);
var test=$("#qty").val();
alert(test);
DEMO

Difference between Element.value and Element.getAttribute("value")

I'm just wondering what the difference is between the two. I have noticed the two methods give different results at times.

The difference is that element.value is real time and if a user changes let's say, a textbox input, it will reflect that, and show you the new value.
While getAttribute('value') will still show the original value="whateverWasHere" value.
jsFiddle DEMO

.value does not map to any attribute.
.defaultValue maps to the "value" attribute. So when you say elem.getAttribute("value") that's the same as elem.defaultValue.
Additionally, .defaultValue reflects .value when the input is untouched (dirty value flag is false). After the input's value is changed by user interaction, this mapping stops. While the input is untouched, you can change .defaultValue (and thus .setAttribute("value")) and see it change .value as well. Not that this is practically useful but interesting piece of trivia nevertheless.