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simplify if else statement

I have some functionality dependent on many conditions. All variables in conditional statements are boolean variables and the code is the following and I don't like it:
if (userHasMoreThanOneMarket && isOnlyMarketSelected || !userHasMoreThanOneMarket && userHasMoreThanOneAgency) {
if (isOnlyAgencySelected) {
//do case 1
} else if (noAgencySelected && isOnlyMarketSelected) {
//do case 2
}
}
Is there a way to make it more understandable and nice?

That's about as concise as you're going to get with JavaScript. I suppose if you really wanted to, you could create variables to store your binary options:
var multiMarketOneSelected = userHasMoreThanOneMarket && isOnlyMarketSelected;
var singleMarketMultiAgency = !userHasMoreThanOneMarket && userHasMoreThanOneAgency;
if (multiMarketOneSelected || singleMarketMultiAgency) {
if (isOnlyAgencySelected) {
//do case 1
} else if (noAgencySelected && isOnlyMarketSelected) {
//do case 2
}
}
Though I don't really know if you gain much readability from that.

Your code seems fine, but if you don't like it you could do something like this (note that the only improvement here is style, if you like it better):
function check(){
return {
valid: userHasMoreThanOneMarket && isOnlyMarketSelected || !userHasMoreThanOneMarket && userHasMoreThanOneAgency,
case: [
isOnlyAgencySelected,
noAgencySelected && isOnlyMarketSelected
]
};
}
var conditions = check();
if (conditions.valid) {
if (conditions.case[0]) {
//do case 1
} else if (conditions.case[1]) {
//do case 2
}
}

Some things I would try to make the code more readable:
Initialise the variables in a way that you don't have to negate them again. So !userHasMoreThanOneMarket becomes userHasOneMarket
isOnlyMarketSelected sounds redundant to me. And you are checking it in the outer if-clause and the inner again.
You probably have a lot of code above this code snippet to initialise and set all this boolean values. Try return; statements after each variable to get rid of if-conditions.
I hope this helps.

"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 = "";
}

Check exactly one boolean option set

Well, this is kind of hacky:
function b2n(boo) {
return boo ? 1 : 0;
}
if(b2n(opt1) + b2n(opt2) + b2n(opt3) !== 1) {
throw new Error("Exactly one option must be set");
}
Is there a better way to do this in Javascript? Using any of
more intelligent boolean/number handling
sneaky array or functional operations
And so forth. Javascript and Node solutions welcome.
In my actual problem, the options are coming from the Node module commander, so I'm not dealing with true boolean, just truthy and falsy things. There may be a commander-solution too.

Assuming you had an array of options, you could do:
if(opts.filter(Boolean).length !== 1) {}
It seems to me though that you ought to have one variable with three possible states instead...
var opt = 'a'; // (or 'b', or 'c')

You can do this :
if ( !!opt1 + !!opt2 + !!opt3 !== 1 ) {
It works because
!! makes a boolean from any value (true if the objects evaluates as true in if(value))
when adding booleans you get 1 for true and 0 for false.

You mentioned in your comment that this is coming from a commander options object.
You can do this more elegantly using Lodash:
if (_(options).values().compact().size() === 1)
If you only want to count a subset of the options, you can insert
.pick('a', 'b', 'c')

if ([opt1, opt2, opt3].reduce(function(x, y) { return x + !!y }, 0) == 1) {
// exactly one
};
ECMAScript 5 reduce function.

I think you are being too clever, what's wrong with:
var optionsSelected = 0;
if( opt1 ) optionsSelected++;
if( opt2 ) optionsSelected++;
if( opt3 ) optionsSelected++;
if( optionsSelected !== 1 ) {
throw new Error("Exactly one option must be set");
}
Of course I can play the clever game too:
if( opts.filter(Boolean).length !== 1 ) {
throw new Error("Exactly one option must be set");
}

#spudly is on the right track, but it could be a little more compact:
if( [opt1,opt2,opt3].filter(function(x){return x}).length!==1 ) {
throw new Error("Exactly one option must be set");
}
See ES5's filter method for more information.

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");
}

Javascript OR in an IF statement

I am trying to make an if statement in javascript that will do something if the variable does not equal one of a few different things. I have been trying many different variations of the OR operator, but I cant get it to work.
if(var != "One" || "Two" || "Three"){
// Do Something
}
Any ideas? Thanks!
Update:
I have tried this before:
if(var != "One" || var != "Two" || var != "Three"){
// Do Something
}
For some reason it does not work. My variable is pulling information from the DOM i dont know if that would effect this.
Actual Code
// Gets Value of the Field (Drop Down box)
var itemtype = document.forms[0].elements['itemtype' + i];
if(itemtype.value != "Silverware" || itemtype.value != "Gold Coins" || itemtype.value != "Silver Coins"){
// Do Something
}

Your expression is always true, you need:
if(!(myVar == "One" || myVar == "Two" || myVar == "Three")) {
// myVar is not One, Two or Three
}
Or:
if ((myVar != "One") && (myVar != "Two") && (myVar != "Three")) {
// myVar is not One, Two or Three
}
And, for shortness:
if (!/One|Two|Three/.test(myVar)) {
// myVar is not One, Two or Three
}
// Or:
if (!myVar.match("One|Two|Three")) {
// ...
}
More info:
De Morgan's Laws
Edit: If you go for the last approaches, since the code you posted seems to be part of a loop, I would recommend you to create the regular expression outside the loop, and use the RegExp.prototype.test method rather than String.prototype.match, also you might want to care about word boundaries, i.e. "noOne" will match "One" without them...

Assuming you mean "val does not equal One or Two or Three" then De Morgan's Theorem applies:
if ((val != "One") && (val != "Two") && (val != "Three")) {
// Do something...
}

For a shorter way to do it, try this format (copied from http://snook.ca/archives/javascript/testing_for_a_v):
if(name in {'bobby':'', 'sue':'','smith':''}) { ... }
or
function oc(a)
{
var o = {};
for(var i=0;i<a.length;i++)
{
o[a[i]]='';
}
return o;
}
if( name in oc(['bobby', 'sue','smith']) ) { ... }

The method mentioned by Mike will work fine for just 3 values, but if you want to extend it to n values, your if blocks will rapidly get ugly. Firefox 1.5+ and IE 8 have an Array.indexOf method you can use like so:
if(["One","Two","Test"].indexOf(myVar)!=-1)
{
//do stuff
}
To support this method on IE<=7, you could define a method called Array.hasElement() like so:
Array.prototype.hasElement = function hasElement(someElement)
{
for(var i=0;i<this.length;i++)
{
if(this[i]==someElement)
return true;
}
return false;
}
And then call it like so:
if(!["One","Two","Three"].hasElement(myVar))
{
//do stuff
}
Note: only tested in Firefox, where this works perfectly.

In addition to expanding the expression into three clauses, I think you'd better name your variable something other than var. In JavaScript, var is a keyword. Most browsers aren't going to alert you to this error.

Alternate way using an array:
var selected = ['Silverware', 'Gold Coins', 'Silver Coins'];
if ( selected.indexOf( el.value ) != -1 ) {
// do something if it *was* found in the array of strings.
}
Note: indexOf isnt a native method, grab the snippet here for IE:
https://developer.mozilla.org/En/Core_JavaScript_1.5_Reference/Objects/Array/IndexOf