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what is the order of boolean logic in Javascript?

I wanted to use two Not and one and in booleans to test if the variable is neither upper case nor lower case.
I used this code so far but it didn't work as required:
else if ((x[i]) !== (x[i].toUpperCase()) && (x[i]!== x[i].toLowerCase()) ){
x.splice(x[i], 1);
}
This code was for a function that sorts entered strings yet uppercase are sorted first.
Here is the full code, I am also open to understanding better ways to create this function apart from boolean logic and the array methods I used.
function alpha(str){ // United States
var x = str.split(""); // [U,n,i,t,e,d,S,t,a,t,e,s]
var cap = [];
var small = [];
for (var i = 0; i<x.length; i++){
if (x[i] == x[i].toUpperCase()){
cap.push(x[i]);
}
else if ((x[i]) !== (x[i].toUpperCase()) && (x[i]!== x[i].toUpperCase()) ) {
x.splice(x[i], 1);
}
else {small.push(x[i]);}
}
var z = cap.sort();
var y = small.sort();
return z.concat(y).join("");
}
Please note the second else if statement is only useful because the code adds an empty space string at the beginning of the output, I'm not sure where it comes from, so please let me know if you have any idea how to sort this even without using the second else if.

In the ASCII table, upper case letters come first. That's why they come first when you sort alphabetically. Here's a link to a page on Wikipedia that shows the table with the upper case letters appearing first and their numerical equivalents. It's even printable.
Also, I took the liberty of simplifying your code a little. Seems like .splice() was not necessary.
function alpha( str ) {
var x = str.split(""); // [U,n,i,t,e,d,S,t,a,t,e,s]
var cap = [];
var small = [];
var length = x.length;
for (var i = 0; i < length; i++) {
if (x[i] === x[i].toUpperCase()) {
cap.push(x[i]);
} else if (x[i] === x[i].toLowerCase()) {
small.push(x[i]);
}
}
return cap.sort().concat(small.sort()).join("");
}
Maybe explain what you're trying to do? It most likely has been done before in some form and you definitely came to the right place to find an answer.

Is this what you want to do?
var str = "United States";
function alpha(str) {
return str.split('').sort().join('');
}
alert(alpha(str));

In all programming languages (as far as i know), boolean expressions are always evaluated from the left to the right with brackets of course.
So in the following example my_func() is called first, and then if there is the chance that the complete expression becomes true my_other_func() is called
if (my_func() && my_other_func()) {
// I only get here if my_func() AND my_other_func() return true
// If my_func() returns false, my_other_func() is never called
}
The same is true for the "or" operator in the following example
if (my_func() || my_other_func()) {
// I only get here if my_func() OR my_other_func() return true
// If my_func() returns true, my_other_func() is not called
}
So back to your code, in details this part (I reformated it a bit for better readability):
if (x[i] == x[i].toUpperCase()){
// only uppercase here
cap.push(x[i]);
} else if (x[i] !== x[i].toUpperCase() && x[i] !== x[i].toUpperCase()) {
// tested twice the same thing, so Im really sure that its not uppercase :D
// only lowercase here
x.splice(x[i], 1);
} else {
// I will never reach this
small.push(x[i]);
}
Im not sure what you want to do, but I hope the comments help to understand your code.

Better method of checking a bunch of conditions

I'm new to javascript and still coming to terms with the language's nuances.
I have a piece of code where I have to check a set of conditions on a particular variable.
if (a=="MAIN_DOMAINNAME" || a=="DOMAIN_SERIAL" || a=="DOMAIN_REFRESH" || a=="DOMAIN_RETRY" || a=="DOMAIN_EXPIRE" || a=="DOMAIN_NEGTTL" || a=="MAIN_NS") {
Is there a better way to do this conditional check, like say:
if a is one of ("DOMAIN_SERIAL", "MAIN_DOMAINNAME", "DOMAIN_REFRESH" ) {?

Assuming a relatively modern browser, you can use Array.indexOf (spec)
if (["DOMAIN_SERIAL", "MAIN_DOMAINNAME", "DOMAIN_REFRESH"].indexOf(a) !== -1)
Note - you can easily shim it for older browsers (see the mdn link on how).

A regex would be shorter and works everywhere :
if ( /^(MAIN_DOMAINNAME|DOMAIN_SERIAL|DOMAIN_REFRESH|..)$/.test(a) ) {
// do stuff
}
FIDDLE

var ars = ["DOMAIN_SERIAL", "MAIN_DOMAINNAME", "DOMAIN_REFRESH"];
if(ars.some(function(ar){ return a === ar; })){
// do smth
}

Should mention the switch statement as it should be working fine with the example given in the question.
switch(a) {
case('MAIN_DOMAINAME'):
case('DOMAIN_SERIAL'):
case('DOMAIN_REFRESH'):
case('DOMAIN_RETRY'):
console.log('Go wild.');
break;
}
Not as lightweight as the other answers, but it's readable and matches (a === b).

I prefer the regex solution already provided by adeneo, but if you want something that matches the
if a is one of (...
wording from the question reasonably closely you can do this:
if (a in list("MAIN_DOMAINNAME", "DOMAIN_SERIAL", "DOMAIN_REFRESH", "DOMAIN_RETRY")) {
// do something (rest of list omitted to avoid scrolling)
}
by providing a helper function to turn the list into an object:
function list() {
var o={}, i;
for (i=0; i < arguments.length; i++) o[arguments[i]] = true;
return o;
}
Of course you can omit the helper function and just use an object literal, but that's ugly:
if (a in {"MAIN_DOMAINNAME":1, "DOMAIN_SERIAL":1, "DOMAIN_REFRESH":1}) {

Multiple OR operators with elem.value.match

I've been writing a javascript function which returns true if the value matches one of about 4 values (just 3 in the example below). The problem is, when I have just two values the function works correctly, but adding a third breaks the code.
I'm pretty new to javascript and I'm guessing there's a much better way of doing this? I've tried searching but found nothing as of yet.
Any help is much appreciated.
function isValid(elem, helperMsg){
var sn6 = /[sS][nN]6/;
var sn5 = /[sS][nN]5/;
var sn38 = /[sS][nN]38/;
if(elem.value.match(sn6 || sn5 || sn38)){
//do stuff
return true;
}else{
return false;
}
}
Edit:
Here's my second attempt with an array:
function isLocal(elem, helperMsg){
var validPostcodes=new Array();
validPostcodes[0]= /[wW][rR]12/;
validPostcodes[1]= /[cC][vV]35/;
validPostcodes[2]= /[sS][nN]99/;
validPostcodes[3]= /[sS][nN]6/;
validPostcodes[4]= /[sS][nN]5/;
validPostcodes[5]= /[sS][nN]38/;
validPostcodes[6]= /[oO][xX]29/;
validPostcodes[7]= /[oO][xX]28/;
var i = 0;
for (i = 0; i < validPostcodes.length; ++i) {
if(elem.value.match(validPostcodes[i])){
// do stuff
return true;
}else{
alert(helperMsg);
elem.focus();
return false;
}
}
}

a || b || c
is an expression that evaluates to a boolean. That means that you're running either match(true) or match(false). You must write it as:
match(a) || match(b) || match(c)
Another option would be to store them in an array and loop over it. That would mean if the number of patterns grew you wouldn't have to change code other than the list of patterns. Another approach, though limited to this situation, might be to change the pattern to one that is equivalent to or-ing the three options together (untested, and I'm a bit rusty on regex):
elem.value.match(/[sSnN][6|5|38]/)
Array based example:
var patterns = [/../, /.../];
for (var i = 0; i < patterns.length; ++i) {
if (elem.value.match(patterns[i])) { return true; }
}
In real code, I would probably format it like this:
function isValid(elem, helperMsg){
var patterns = [/../, /.../],
i = 0;
for (i = 0; i < patterns.length; ++i) {
if (elem.value.match(patterns[i])) {
return true;
}
}
}
That's just a habit though since JavaScript hoists variables to the top of their scope. It's by no means required to declare the variables like that.

JavaScript/jQuery equivalent of LINQ Any()

Is there an equivalent of IEnumerable.Any(Predicate<T>) in JavaScript or jQuery?
I am validating a list of items, and want to break early if error is detected. I could do it using $.each, but I need to use an external flag to see if the item was actually found:
var found = false;
$.each(array, function(i) {
if (notValid(array[i])) {
found = true;
}
return !found;
});
What would be a better way? I don't like using plain for with JavaScript arrays because it iterates over all of its members, not just values.

These days you could actually use Array.prototype.some (specced in ES5) to get the same effect:
array.some(function(item) {
return notValid(item);
});

You could use variant of jQuery is function which accepts a predicate:
$(array).is(function(index) {
return notValid(this);
});

Xion's answer is correct. To expand upon his answer:
jQuery's .is(function) has the same behavior as .NET's IEnumerable.Any(Predicate<T>).
From http://docs.jquery.com/is:
Checks the current selection against an expression and returns true, if at least one element of the selection fits the given expression.

You should use an ordinary for loop (not for ... in), which will only loop through array elements.

You might use array.filter (IE 9+ see link below for more detail)
[].filter(function(){ return true|false ;}).length > 0;
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter

I would suggest that you try the JavaScript for in loop. However, be aware that the syntax is quite different than what you get with a .net IEnumerable. Here is a small illustrative code sample.
var names = ['Alice','Bob','Charlie','David'];
for (x in names)
{
var name = names[x];
alert('Hello, ' + name);
}
var cards = { HoleCard: 'Ace of Spades', VisibleCard='Five of Hearts' };
for (x in cards)
{
var position = x;
var card = card[x];
alert('I have a card: ' + position + ': ' + card);
}

I suggest you to use the $.grep() method. It's very close to IEnumerable.Any(Predicate<T>):
$.grep(array, function(n, i) {
return (n == 5);
});
Here a working sample to you: http://jsfiddle.net/ErickPetru/BYjcu/.
2021 Update
This answer was posted more than 10 years ago, so it's important to highlight that:
When it was published, it was a solution that made total sense, since there was nothing native to JavaScript to solve this problem with a single function call at that time;
The original question has the jQuery tag, so a jQuery-based answer is not only expected, it's a must. Down voting because of that doesn't makes sense at all.
JavaScript world evolved a lot since then, so if you aren't stuck with jQuery, please use a more updated solution! This one is here for historical purposes, and to be kept as reference for old needs that maybe someone still find useful when working with legacy code.

Necromancing.
If you cannot use array.some, you can create your own function in TypeScript:
interface selectorCallback_t<TSource>
{
(item: TSource): boolean;
}
function Any<TSource>(source: TSource[], predicate: selectorCallback_t<TSource> )
{
if (source == null)
throw new Error("ArgumentNullException: source");
if (predicate == null)
throw new Error("ArgumentNullException: predicate");
for (let i = 0; i < source.length; ++i)
{
if (predicate(source[i]))
return true;
}
return false;
} // End Function Any
Which transpiles down to
function Any(source, predicate)
{
if (source == null)
throw new Error("ArgumentNullException: source");
if (predicate == null)
throw new Error("ArgumentNullException: predicate");
for (var i = 0; i < source.length; ++i)
{
if (predicate(source[i]))
return true;
}
return false;
}
Usage:
var names = ['Alice','Bob','Charlie','David'];
Any(names, x => x === 'Alice');

Easy way to evaluate path-like expressions in Javascript?

If I have a JavaScript object such as:
var x = {foo: 42, bar: {fubar: true}}
then I can get the value true with var flag = x.bar.fubar. I'd like to be able to separate out and store the path "bar.fubar", then evaluate it dynamically. Something like:
var paths = ["bar.fubar", ...];
...
var flag = evalPath( x, paths[0] );
Obviously I could write a simple parser and evaluator for a basic path expression grammar. But under DRY principles I wonder if there's already an existing way to do something like evalPath built-in to JavaScript, or a small library that would do the job? I also anticipate needing array indexes in the path expression in future.
Update Just to be clear, I'm not asking for code samples - my question is whether there's existing code (built-in or library) I can re-use. Thanks to the contributors below for suggestions of code samples anyway! Note that none of them handle the array index requirement.

Doing a quick search, I came across JSONPath. Haven't used it at all, but it looks like it might do what you want it to.
Example usage:
var x = {foo: 42, bar: {fubar: true}}
var res1 = jsonPath(x, "$.bar.fubar"); // Array containing fubar's value

Why not try something like
function evalPath(obj, path)
{
var rtnValue = obj;
// Split our path into an array we can iterate over
var path = path.split(".");
for (var i = 0, max=path.length; i < max; i++)
{
// If setting current path to return value fails, set to null and break
if (typeof (rtnValue = rtnValue[path[i]]) == "undefined")
{
rtnValue = null;
break;
}
}
// Return the final path value, or null if it failed
return rtnValue;
}
Not tested, but it should work fairly well. Like XPath, it will return null if it can't find what it's looking for.

JavaScript provides eval, but I don't recommend it.

like
function locate(obj, path) {
var p = path.split("."), a = p.shift();
if(a in obj)
return p.length ? locate(obj[a], p.join(".")) : obj[a];
return undefined;
}
locate(x, "bar.fubar")
this works on the right only, of course

You could try something like this. I can't really think of a situation where it would be appropriate to store paths this way though.
function evalPath(obj, path) {
var pathLevels = path.split('.');
var result = obj;
for (var i = 0; i < pathLevels.length; i++) {
try {
result = result[pathLevels[i]];
}
catch (e) {
alert('Property not found:' + pathLevels[i]);
}
}
return result;
}
The alert is really only there for debugging purposes. You may want to return null or something.

How about:
evalPath = function(obj, path) {
if (path[0] === "[") {
return eval("obj" + path);
} else {
return eval("obj." + path);
}
};
This has the advantage that it works for arbitrary strings:
evalPath([1,2,3], "[0]"); => 1
evalPath({a:{b:7}}, "a.b"); => 7
This, of course, only works if you really trust your input.