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Adobe Acrobat 2017 Javascript is null?

In this code:
this.getField("myField").value == null;
this.getField("myField").value === null;
typeof this.getField("myField").value == null;
typeof this.getField("myField").value === null;
this.getField("myField").rawValue === null;
this.getField("myField").formattedValue === "";
this.getField("myField").isNull;
this.getField("myField").isNull == True;
all of the above exchanging 'null' for 'Null', encapsulated 'Null', and 'undefined'.
In each circumstance all I get is:
TypeError: this.getField(...) is null
23:Field:Blur
How do I see if a field is null? I do not want to have default values because not every field on the form needs to be used and should be able to be blank.

If you're getting that error, it's because this.getField("myField") itself is returning null. So any attempt to use a property on what it returns will fail.
It sounds like you need a null guard:
var field = this.getField("myField");
if (field !== null) }
// use `field.value` here...
}

Generally... meaning you used an application rather than a library to create the form... PDF field values will never be null. An empty field has a zero-length string as the default value. To test if the field is empty use...
if (this.getField("myField").value.length == 0) {
// do something
}
else {
// it has a value
}
or
if (this.getField("myField").value == "") {
// do something
}
else {
// it has a value
}

"If" consolidation/avoiding nesting

I'm really trying to avoid nesting in this code snippet...
deal_trade_in_model_1 = document.getElementById('deal_trade_in_model_1').value;
deal_trade_in_amount_1 = document.getElementById('deal_trade_in_amount_1').value;
if (typeof deal_trade_in_model_1 !== 'undefined' && deal_trade_in_model_1 !== null) {
console.log(deal_trade_in_amount_1);
console.log(deal_trade_in_model_1);
if (deal_trade_in_model_1 !== null || deal_trade_in_model_1 !== "") {
if (deal_trade_in_amount_1 == null || deal_trade_in_amount_1 == "") {
console.log('entered into function');
document.getElementById("deal_trade_in_model_1").value = "";
document.getElementById("deal_trade_in_amount_1").value = "";
}
}
}
Basically, what this function does is take the value of two fields... things to know about them and what I want to do to them:
1) They're NOT required
2) If one of them is filled out, the other must be
3) If ONLY one of them is filled out, the user clicks submit, and this part of the function is called upon, I want to delete the value of both of them.
I've tried doing a compound of
&& (and)
and
|| (or)
buttttt it odiously it didn't work.
Primary question: What's the best way to get rid of the nesting (I planned on doing this twice and just swapping the code) that will be the most efficient? This, I want, to be done preferably in the smallest amount of IF statements possible.
Please note: If you change the code a lot, I might not know what you're talking about.. please be prepared to teach me or help me learn!

It sounds like you only want to do something if either of the fields are empty, but not both. Assuming both of the elements are text fields, .value will always return a string. Converting a string to boolean results in false if the string is empty, otherwise true.
So
Boolean(deal_trade_in_model_1) === Boolean(deal_trade_in_amount_1)
will be true if either both fields have a value (both will convert to true) or both fields are empty (both convert to false).
Thus your code can be reduced to
var model_1 = document.getElementById('deal_trade_in_model_1');
var amount_1 = document.getElementById('deal_trade_in_amount_1');
if (Boolean(model_1.value) !== Boolean(amount_1.value)) {
model_1.value = "";
amount_1.value = "";
}

Variable not updating its value

var canAssignMultiple="true";
var canWithdrawMultiple="true";
function onCheckUncheck()
{
if($(':checkbox[name^="checkedRecords"]:checked').length>0)
{
$("input[name='checkedRecords']:checked").each(function()
{
debugger;
var canAssign = $(this).attr("canAssign").toLowerCase();
var canWithdraw = $(this).attr("canWithdraw").toLowerCase();
canAssignMultiple= canAssignMultiple && canAssign;
canWithdrawMultiple= canWithdrawMultiple && canWithdraw;
if (canAssignMultiple == "false")
$("#assaignbutton").attr("disabled", "disabled");
else
$("#assaignbutton").removeAttr("disabled");
if (canWithdrawMultiple == "false")
$("#withdrawbutton").attr("disabled", "disabled");
else
$("#withdrawbutton").removeAttr("disabled");
});
}
else
{
$("#assaignbutton").attr("disabled", "disabled");
$("#withdrawbutton").attr("disabled", "disabled");
}
}
The variable canAssignMultiple is becoming true when each() function is called the second time though its value has changed to false in the first iteration.It should retain its value evrytime the loop runs.How to do this?

boolean (false, true) values are different than strings ("false", "true")
try
var canAssignMultiple = true;
var canWithdrawMultiple = true;
function onCheckUncheck() {
if ($(':checkbox[name^="checkedRecords"]:checked').length > 0) {
$("input[name='checkedRecords']:checked").each(function() {
debugger;
var canAssign = $(this).attr("canAssign").toLowerCase() == "true"; // make this a boolean expression
var canWithdraw = $(this).attr("canWithdraw").toLowerCase() == "true"; // make this a boolean expression
canAssignMultiple = canAssignMultiple && canAssign;
canWithdrawMultiple = canWithdrawMultiple && canWithdraw;
if (canAssignMultiple === false) $("#assaignbutton").attr("disabled", "disabled"); // use false (instead of "false")
else $("#assaignbutton").removeAttr("disabled");
if (canWithdrawMultiple === false) $("#withdrawbutton").attr("disabled", "disabled"); // use false (instead of "false")
else $("#withdrawbutton").removeAttr("disabled");
});
}
else {
$("#assaignbutton").attr("disabled", "disabled");
$("#withdrawbutton").attr("disabled", "disabled");
}
}​
left comments where changes were made

As others have noted, the strings "true" and "false" do not function the same as the boolean values true and false. If you try to use a string in an if statement or with a logical operator such as &&, the string is converted to a boolean; and all non-empty strings are converted to true (i.e. they are "truthy"). That means the string "false" will work the opposite of how you expect, e.g. in ("false" && x) or in if ("false") {...}.
One way to fix your code is to change this line
canAssignMultiple = canAssignMultiple && canAssign;
to convert canAssign to a boolean in an explicit and correct way:
canAssignMultiple = canAssignMultiple && (canAssign == "true");
Then the && operator will work correctly, and canAssignMultiple will hold an actual boolean value.
And (as #mdmullinax noted, but I would do it differently) once canAssignMultiple is actually a boolean, you can change your if statements to treat them as such:
if (canAssignMultiple == "false")
should become
if (!canAssignMultiple)
And similarly with the corresponding withdraw stuff.

Does your HTML look like this?
<input canAssign="false" ...>
<input canAssign="true" ...>
If so, and you can't change the HTML, I suggest changing this:
canAssign = $(this).attr("canAssign").toLowerCase()
to this (to work with booleans from then on):
canAssign = $(this).attr("canAssign") == "true"
and this:
if(canAssignMultiple == "false")
to this (since it is a real boolean now):
if(canAssignMultiple)
If you can change HTML, I suggest you do it the HTML5-compliant way. First, all of your made up attributes should be prefixed data-. You can also change camelCase to hyphen-case. JQuery, when loading your data-attributes, will then perform the conversion from string if possible:
<input data-can-assign="false" ...>
<input data-can-assign="true" ...>
then jQuery will give you the boolean you want from this call:
canAssign = $(this).data("canAssign")
Of course, the same fix should be applied to all other string pseudo-booleans (that don't quite work with the boolean operators) as well.
The reason is: "false" is not exactly false. "false" && x will evaluate to x (not to "false" as you expect). On the other hand, false && x will evaluate to false.

How to set a var to true or false using .val()

I have a textbox, I need to check if the textbox is empty and set a variable accordingly to true or false.
my code at the moment print the value of the textbox.
var isDateTimEndEmpty = dateTimeEnd.val() == null ? true : false;
what am I doing wrong here?

Here is the correct way:
var isDateTimEndEmpty = !$.trim(dateTimeEnd.val()).length;
Good practice is to use $.trim which makes the code to ignore spaces in the input field. So if the length of trimmed value is 0 then the value is empty.

Check its length
var isDateTimeEndEmpty = $.trim($('#txtboxID').val()).length > 0;

the result of dateTimeEnd.val() won't be null. it always a string, might be empty string.
in actually, you don't need to get a 'true' of 'false', just a "truly" or "falsy" value, like empty or non-empty string:
var isDateTimEndEmpty = $.trim(dateTimeEnd.val()) //remove spaces at start and end
if (isDateTimEndEmpty) {
//if dataTimeEnd is not empty, go here
} else {
//if dateTimeEnd is empty, go here
}
if you were uncomfortable with the implicit casting, you can code it in explicit way as well:
var isDateTimEndEmpty = Boolean($.trim(dateTimeEnd.val())); //explicit casting

Faster and shorter way to check if a cookie exists

What is the shorter and faster way to know if a cookie has a value or exists?
I'm using this to know if exists:
document.cookie.indexOf('COOKIENAME=')== -1
This to know if has a value
document.cookie.indexOf('COOKIENAME=VALUE')== -1
Any better? Any problems on this method?

I would suggest writing a little helper function to avoid what zzzzBov mentioned in the comment
The way you use indexOf, it would only evaluate correct if you check for the containment of a String in a cookie, it doesn't match a complete name, in that case the above would return false therefore giving you the wrong result.
function getCookie (name,value) {
if(document.cookie.indexOf(name) == 0) //Match without a ';' if its the firs
return -1<document.cookie.indexOf(value?name+"="+value+";":name+"=")
else if(value && document.cookie.indexOf("; "+name+"="+value) + name.length + value.length + 3== document.cookie.length) //match without an ending ';' if its the last
return true
else { //match cookies in the middle with 2 ';' if you want to check for a value
return -1<document.cookie.indexOf("; "+(value?name+"="+value + ";":name+"="))
}
}
getCookie("utmz") //false
getCookie("__utmz" ) //true
However, this seems to be a bit slow, so giving it an other approach with splitting them
Those are two other possibilities
function getCookie2 (name,value) {
var found = false;
document.cookie.split(";").forEach(function(e) {
var cookie = e.split("=");
if(name == cookie[0].trim() && (!value || value == cookie[1].trim())) {
found = true;
}
})
return found;
}
This one, using the native forEach loop and splitting the cookie array
function getCookie3 (name,value) {
var found = false;
var cookies = document.cookie.split(";");
for (var i = 0,ilen = cookies.length;i<ilen;i++) {
var cookie = cookies[i].split("=");
if(name == cookie[0].trim() && (!value || value == cookie[1].trim())) {
return found=true;
}
}
return found;
};
And this, using an old for loop, which has the advantage of being able to early return the for loop if a cookie is found
Taking a look on JSPerf the last 2 aren't even that slow and only return true if theres really a cookie with the name or value, respectively
I hope you understand what i mean

Apparently:
document.cookie.indexOf("COOKIENAME=VALUE");
For me, is faster, but only slightly.
As the test shows, surprisingly, it's even faster to split the cookie up into arrays first:
document.cookie.split(";").indexOf("COOKIENAME=VALUE");

I use Jquery cookie plugin for this.
<script type="text/javascript" src="jquery.cookie.js"></script>
function isCookieExists(cookiename) {
return (typeof $.cookie(cookiename) !== "undefined");
}