Develop Reference

Why checking undefined type instead of just value?

I got recently such legacy code:
if (typeof value != "undefined" && value.someOperation()) { }
I understand that its preventing undefined object issue, however isn't simple to do such thing:
if (value && value.someOperation()) { }
Is somewhere in deep javascript a hack or some situation that it would not work at all? There is no possible to get 0 or false instead of that object here. I wonder if I can change the first expression to the second one and I wouldn't break anything.

In this line
if (typeof value != "undefined" && value.someOperation()) { }
typeof value != "undefined" is evaluated to false if and only if value is either null or undefined. However, if the value is either 0 or false then it will evaluate to true, and hence move on to second condition value.someOperation().
There is no possible to get 0 or false instead of that object here.
If the value of value cannot be 0 or false, even then you might want to check value.someOperation before checking value.someOperation() since value.someOperation() it may give following error
TypeError: undefined is not a function
In fact, in both cases you might want to check if value.someOperation first before value.someOperation() i.e.
if (value && value.someOperation && value.someOperation()) { }
or
if (typeof value != "undefined" && value.someOperation && value.someOperation()) { }

How can this conditional be shortened?

if (!this.initialized || (typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId)) {
//Stuff
}
I want to execute code if the this.initialized flag is false or this.activeId value isn't the same as options.qualification_id. I have to check if this.activeId is defined or not because Javascript will return true in the case of undefined !== this.activeId. The above is the conditional I've come up with, but I hate how long it is. What would be the shortest way to write it?

If the valid values for this.activeId when it's defined are all truthy (e.g. it can't be 0, false, or null), you can use:
if (!this.initialized || this.activeId && options.qualification_id != this.activeId)

Let's see what you can do to shorten this
(!this.initialized || (typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId))
Well first, you can remove the inner parenthesis. According to the operator precedence table, logical AND has a "higher precedence" than logial OR. In other words the parenthesis will be added for you by the interpreter. So, your condition becomes:
(!this.initialized || typeof this.activeId !== 'undefined' && options.qualification_id !== this.activeId)
Not much better. Another thing you can do is take advantage of JS type conversion. undefined values are falsey (are evaluated to false), your condition can become:
(!this.initialized || this.activeId && options.qualification_id !== this.activeId)
However, this may insert a silent bug since an activeId of 0 will also be evaluated to false. You can solves this using the in operator which checks to see if the property exists on the object or on one of its prototypes (To make sure the property doesn't come from one of its prototypes you can use the hasOwnProperty method instead).
(!this.initialized || 'activeId' in this && options.qualification_id !== this.activeId).
Now to be honest, this doesn't look so much better to me. As a rule, whenever the line becomes too long, just split it in multiple lines to increase readability.
var qualificationIsDifferent = options.qualification_id !== this.activeId;
var isNotActive = this.hasOwnProperty('activeId') && qualificationIsDifferent;
var shouldDoStuff = !this.initialized || isNotActive;
if ( shouldDoStuff ) { ... }
Note that by using the above approach you lose some of the benefits of the short-circuit evaluation.
It really depends on your goal, sometimes more verbose means more readable.

check for undefined or null

I'm checking if the value of cookie is undefined or null but weirdly this does not really work
Here's what I tried
function check(val){
if($.cookie(val) === "undefined" || $.cookie(val) === "null"){
return "1";
}
return $.cookie(val);
}
So even if my cookie has value null it would print out null instead of 1

That is because you are using quotes. In other words, "undefined" === undefined is false.
if($.cookie(val) === undefined || $.cookie(val) === null){ //this would be more apropriate

Please check if jquery cookie library is included , this is the most general cause.

How to easily check if undefined in javascript object?

Assuming I have a javascript object "myobject" that just contains an empty object "{}".
I ideally in my code I want to do the following:
if (theobject[keyvar][subkeyvar]) {
// do something
}
The issue now is that because keyvar and subkeyvar do not actually exist within the object, it fails and comains that the properties are undefined. What is the simplest/least lines of code/best way to be able to do have it just "know it is undefined" and continue to execute the //do something or not without crashing?
I dont want to get too carried away with checking like:
if( keyvar in theobject ) {
if(subkeyvar in theobject) {
if.....
}
}

if (typeof theobject[keyvar] !== "undefined" && typeof theobject[keyvar][subkeyvar] !== "undefined") {
// keyvar and subkeyvar defined
}
first we check if keyvar typeof is undefined and then if subkeyvar is undefined and if they are both defined typeof theobject[keyvar] !== "undefined" && typeof theobject[keyvar][subkeyvar] !== "undefined" is true.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

you can try this :
if (theobject[keyvar] && theobject[keyvar][subkeyvar]) {
// do something
}

If they both can possibly be undefined, you have to test one, then the other.
if (theobject[keyvar] && theobject[keyvar][subkeyvar]) {
This will of course fail on a key containing 0 or "" (or several other falsey but not undefined values) so you may want to use typeof on the second test.

Try:
if (theobject[keyvar] !== undefined && theobject[keyvar][subkeyvar] !== undefined) {
// do something
}
Since the && operator is used, the if will not check the second value if the first value is falsey, so you will never get the 'properties are undefined` error

theObject.hasOwnProperty(keyvar) && theObject.hasOwnProperty[keyvar].hasOwnProperty(subkeyvar)

How to handle a not defined value in JavaScript

How to handle a not defined value in JavaScript?
if (oldins == ins)
oldins is not defined.
How do I check this?

if ((typeof(oldins) !== "undefined") && (oldins === ins))

Unset variables would evaluate to a value of 'undefined'. 'undefined' is a value type like null and NaN so it would be:
if ( typeof(oldins) == 'undefined' )
Edit: Fixed per comments. Leaving the answer since the comments are helpful, but there were more correct answers.

if (oldins !== undefined && oldins === ins) {
}