Develop Reference

checking for null in javascript

if (valid === null) {
return '';
} else if (!valid) {
return 'is-not-valid';
} else if (valid) {
return 'is-valid';
} else {
return '';
}
I have the above if-else-if chain in the code, trying to see if I can write the same logic in one or two lines.

Since you want to distinguish between three types of values, you have to make at least two checks already. The else case will never be hit since either !valid or valid will be true. That also means you can reduce the last else if to an else:
if (valid === null) {
return '';
} else if (!valid) {
return 'is-not-valid';
} else {
return 'is-valid';
}
But you could condense this logic using the conditional operator:
return valid === null ? '' : (valid ? 'is-valid' : 'is-not-valid');

Although I think your example (sans the redundant else block) looks pretty good, you can write it on one line like this:
return valid === null ? '' : (valid ? 'is-valid' : 'is-not-valid')
I prefer the original though

is this the right way to pull booleans from localstorage?

function loadChoice(key, varToStore) {
if (window.localStorage.getItem(key) === "true") {
varToStore = true;
}
else if (window.localStorage.getItem(key) === "false") {
varToStore = false;
}
else {
varToStore = window.localStorage.getItem(key);
}
}
I think I found a way to pull booleans (that got converted to strings before) from the web-browser localstorage.
can anyone confirm or have a better method ?

true == false evaluates to true somehow?

I've been working to scrape some webpage that is using the OWASP CRSFGuard project for protection. The library seems to be causing one of my requests to get a 401 so I started digging through their code and noticed the following;
function isValidDomain(current, target) {
var result = false;
/** check exact or subdomain match **/
if(current == target || current == 'localhost') {
result = true;
} else if(true == false) {
if(target.charAt(0) == '.') {
result = current.endsWith(target);
} else {
result = current.endsWith('.' + target);
}
}
return result;
}
From what I can tell, there must be instances where this code is executed; result = current.endsWith('.' + target);. Given true == false is inherently false, how would the code reach that statement? Is this some JS oddity (I know we're not using the strict === equality, but seriously...)?

Answer: It will never reach that code block.
function isValidDomain(current, target) {
var result = false;
/** check exact or subdomain match **/
if (current == target || current == 'localhost') {
result = true;
} else if (true == false) {
if (target.charAt(0) == '.') {
result = current.endsWith(target);
} else {
result = current.endsWith('.' + target);
}
}
return result;
}
var trueFalse = document.getElementById('trueFalse');
trueFalse.innerHTML = isValidDomain('true', 'false') ? 'WTF!' : 'All is good in the JS World';
trueFalse.addEventListener('click', function(e) {
trueFalse.innerHTML = (true == false) ? 'WTF!' : 'All is good in the JS World Still';
});
<div id="trueFalse"></div>

I would say that Blazemonger is most likely correct.
That else if probably had some other condition at some point, and for whatever reason, they decided they didn't want that block of code to execute anymore, so they changed the condition to something that is always false.
It's also not entirely uncommon to see programmers use 1 === 0 as an indication for false. Why they would want to do this is anybody's guess.

JavaScript ternary operator into full if/else statement issue

I have following ternary statement:
$.history.init(function(url) {
load(url == "" ? "#some-page" : url);
});
Which I have rewrote into:
$.history.init(function(url) {
load(
if( url == ""){ url = "#some-page"
} else { url = url }
);
});
I now the is an error on line 3 if(url == ""), but I don't understand what error.
Any suggestion much appreciated.

In JavaScript, an if is not an expression. It does not return a value and cannot be put inside a function call. That is, this is not valid:
func(if (a) { ... } else { ... });
This is the main difference between if and ?:--the operator is an expression and returns a value; if is a statement, does not return a value and cannot be used everywhere.
Your best bet if you have to avoid the ternary operator is to do something like:
if (url == "") {
url = "#some-page";
}
load(url);
You can also achieve the same effect using ||:
function (url) {
load(url || "#some-page");
}
This is the shortest and most idiomatic way to write your code.

if expressions dont return anything in JS. So that basically does load(undefined).
Try this instead:
if (url === '') {
url = '#some-page';
}
load(url);
Note you don't need to else at all, because if the value is present you have nothing to change.

rewrite it as
$.history.init(function(url) {
if( url == ""){
url = "#some-page";
}
load( url );
});

Your rewritten code is invalid. Try this:
$.history.init(function(url) {
if(url == "") {
load("#some-page");
} else {
load(url);
}
});

You need the if statement to be outside of the load function, i.e.
$.history.init(function(url) {
if (url === "") {
url = "#some-page";
}
load(url);
});
Note that you don't need the else clause as url = url is a redundant operation.

How can I check if the contents of a variable start with something?

I would like to do something like this:
if (idCity == 'AB*') {
// do something
}
In other words. I want to check that idCity starts with the string "AB". How can I do this in Javascript?

if (idCity.substr(0,2) == 'AB') {
alert('the string starts with AB');
}

if(idCity.indexOf('AB') == 0)
{
alert('the string starts with AB');
}

if(idCity.substr(0,2)=='AB'){
}
If 'AB' is not constant string, you may use
if(idCity.substr(0,start.length)==start){
}

idCity = 'AB767 something';
function startWith(searchString){
if (idCity.indexOf(searchString) == 0){
return true;
}
return false;
}