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JS optional chaining clarification

I found a really great use case for optional chaining in my react project. I have used it slightly before but this line of code has made me a bit confused.
I have a table that is sortable.
One of the columns shows success or failure depending on if failed_date is null or a date.
sortedRows = sortedRows.sort((a, b) => a?.failed_date?.localeCompare(b?.failed_date));
But What confuses me is which value does the '?.' check is nullish?
Does a.failed_date?.localeCompare() check if failed_date?. is null/undefined or does it check if ?.localeCompare() is null/undefined?
Same with b?.failed_date is it checking b to be nullish? or failed_date.
I think My confusion comes from looking at the Documentation
Because arr?.[50] checks if element 50 is nullish but obj.method?.() checks if the method is not nullish?
Looking at this second block of code (which I believe is now correct) a.failed_date may be nullish and won't do localeCompare if a.failed_date is null?
But what if a.failed_date is not null but b.failed_date is null?
Does localeCompare not care? I haven't gotten an error but I was using localeComapre(b?.failed_date)
sortedRows = sortedRows.sort((a, b) => a.failed_date?.localeCompare(b.failed_date));

Let's say you define variable like below
const variable = { value: 'test' };
then you want to access variable?.value it equals variable === null || variable === undefined ? undefined : variable.value.
Same with array.
Check typescript playground and see js output https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXAnjAvDA2gXQNwBQBiyKA-AHRYCsADDkA

Basically, the ? in that context means optional chaining.
How it works is, for example, if you define an object like below, and you want to try and access the views property, it will throw an error.
const obj = { website: "stackoverflow.com", };
console.log(obj.views.toFixed(0)); // Error!
This is because you are trying to access a method off of undefined, which doesn't have anything.
To avoid this error, we can use optional chaining, like below.
const obj = { website: "stackoverflow.com", };
console.log(obj?.views?.toFixed(0)); // undefined
Optional chaining tells JavaScript to complete the method/read the key, but if it doesn't exist, instead of throwing an error, just return undefined.
It also works with arrays; the same way! If, say, index 78 doesn't exist, instead of throwing an error, it will just return undefined.
const arr = [1, 2, 3];
console.log(arr?.[78]?.toString());

To elaborate here, it is possible to stack multiple optional chaining operators as seen in OP's code a?.failed_date?.localeCompare(b?.failed_date)
In these cases, it is not a question of which object key value will be checked. The code will be evaluated from left to right and if any of the object values are nullish then undefined will be returned.
Refer to the documentation for further understanding
MDN Optional Chaining

How to handle JavaScript objects (parsed JSON) that contain null?

I'm using JavaScript/jQuery to fetch data as json via an ajax call. The json is then parsed to a JavaScript object. Let's call the object var.
Now I want to assign a value from the object to a variable:
let publicationDay = val["work-summary"][0]["publication-date"]["day"]["value"]
This works, as long this value exists. However, for some results, the object contains null not for ["value"] but for ["day"] (as shown below)
val["work-summary"][0]["publication-date"]["day"] // is null
In this case, let publicationDay = val["work-summary"][0]["publication-date"]["day"]["value"] will throw the following error:
Uncaught TypeError: val['work-summary'][0]['publication-date'].day is null
and the code execution is stopped. Sure, I can check whether val["work-summary"][0]["publication-date"]["day"] === null, but what would be the most elegant way to handle this case? Maybe, in the future even
val['work-summary'][0]['publication-date'] // is null
may occur and my check for null does not work.

I would recommend trying optional chaining.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining
Assuming that any of those values could be null:
let publicationDay = val?.["work-summary"]?.[0]?.["publication-date"]?.["day"]?.["value"]
It will set the variable to either day.value or undefined

Depends heavily on your context but here’s some examples.
const publicationDate = val['work-summary'][0]['publication-date'];
// usage
console.log(publicationDate && publicationDate.day && publicationDate.day.value); // very basic
console.log(publicationDate?.day?.value); // optional chaining, better if supported by your environment (should be)
console.log(get(publicationDate, “day.value”)); // lodash get()

How to solve cannot read property "length"? [reactjs]

I am trying to get the length of an object but am getting this error message:
Uncaught TypeError: Cannot read property 'length' of undefined
I am trying to get the length and getting a popup when the length is zero.
I tried using this.props.storyboardTargets.length === 0
case 1: data not available so (!this.props.storyboardTargets)--> undefined
case 2: data was there and later deletd or cleared so need to check the length
Here is my code below:
handleShowPopupTarget = () => {
if (this.props.storyboardTargets && !this.props.storyboardTargets.length) {
console.log(this.props.storyboardTargets);
toastWarning(WARNING_MSG_NO_TARGET);
}
};

The way you have it written now does not handle the issue that this.props.storyboardTargets may be undefined. You need to update it like so:
handleShowPopupTarget = () => {
if (!this.props.storyboardTargets || !this.props.storyboardTargets.length) {
console.log(this.props.storyboardTargets);
toastWarning(WARNING_MSG_NO_TARGET);
}
};
This means if storyboardTargets is undefined, or its length is 0, toastWarning will fire.
As an alternative, you could define a default prop for your component of an empty array for storyboardTargets. That way it would never be undefined.

The error message means that storyboardTargets in undefined. Try seeing if storyboardTargets is being passed in to the component that contains your handleShowPopupTarget method.

use lodash's get, it simplifies a lot those kind of buggy checks:
_.get(this.props, 'storyboardTargets.length', 'default'); // you could use 0 instead of 'default' in your case

Your original code was
if (!this.props.storyboardTargets.length) {
And it fails because this.props.storyboardTargets is undefined and well you can not read properties of something that is undefined so it throws an error.
So after that you listened to advice and changed your code to be
if (this.props.storyboardTargets && !this.props.storyboardTargets.length)
So now this stops the undefined error from happening on this line because the truthy check on this.props.storyboardTargets prevents the evaluation of the second half of the code. So that means the code will not go into the if. but you WANT it to go into the if statement if it is not defined.
So what you need to do is change it to be an OR check so if it is NOT defined OR it does not have a length
if (!this.props.storyboardTargets || !this.props.storyboardTargets.length)
Now it goes into the if statement id it is undefined and will not throw the error.
The other solution is to see that it is undefined and set it to a default value
this.props.storyboardTargets = this.props.storyboardTargets || []
if (!this.props.storyboardTargets.length)
Now if the array is undefined, it sets it to an empty array and the if check will work correctly. Changing the data might not be the best solution if other things rely on undefined.

var x; ... x.trim(); Why sometimes it allows but sometimes it makes the rest of the code stop working?

This minor issue causes me 5 hours to fix. Finally I figured out. See this code:
<script language="JavaScript" type="text/javascript">
var x;
.... // a lot of codes here
var k=x.trim();
</script>
The above code made the whole app stop working!
I remembered that I used to do like that before but got no problem.
So, about var x; ... x.trim();, Why sometimes it allows but sometimes it makes the rest of the code stop working?
And what is the best code practice for it?

You can do like this:
if(typeof x === 'undefined'){
// your get an error message
}
else
{
var k=x.toString().trim();
}
Using strict equality operator === above is good idea there because in JS, you can name a variable as undefined too:
var undefined = "something";
So using === makes sure that you are really checking against undefined value for a variable.

trim is a function of String. Refer MDN - String.trim().
So when you apply it to an integer, it fails and throws error, causing you code to stop work
Example
try{
var a = 1;
console.log(a.trim());
}
catch(ex){
console.log(ex);
}
You can try to convert number to string using .toString() and then apply .trim()
try{
var a = 1;
console.log(a.toString().trim());
}
catch(ex){
console.log(ex);
}

I would expand Rajesh's answer. He's right, when you try to call a method that does not exist, a TypeError is thrown. The easiest and fool-proof approach would be to use try/catch to ensure that the rest of the code would be executed as it should. But it's likely that even if it does, you don't get the result you want.
I believe the best way to do would be to wrap the value you're having into String object. It's as easy as
var k = String(x).trim();
It does several important things:
Converts the value of x, whatever it be, into a string, i.e. when you check its type, it's always 'string' and is always an instance of the String object.
Ensures that the resulting value has the method trim which does what it should.
Doesn't throw any error, so the rest of the code is executed.
There may be several pitfalls. If x is undefined, null, NaN or an object, the result of String(x) would be, correspondingly, 'undefined', 'null', 'NaN', or '[object Object]'. If x is an array, it's a specific case, and the value would be the same as if you call x.join(','), for example
x = [1, 2, 3];
var k = String(x).trim; // k is now '1,2,3'
So always keep in mind what types you're dealing with.
Just as with String, you can cast variables to other types, but naïvely converting anything into a Number, a String or an Array is considered a very bad practice. You should always be somewhat sure what type you're working with.

JSON key exists but returns false

First of all, all my code is done in node.js but this can all be applied to javascript too.
This is my code I use to check if the keys exist, the problem is that it always returns false. So I added in the console.log to shows what the values are:
if(!choice.name || !choice.realm || !choice.region || !choice.roll){
console.log(choice);
console.log(choice.name);
console.log(choice.realm);
console.log(choice.region);
console.log(choice.roll);
return false;
}
This is the output of that:
{"name":"Imacactus","realm":"Velen","region":"US","roll":"DPS"}
undefined
undefined
undefined
undefined
I'm guessing it has something to do with the quotes? but I've never heard of quotes messing it up. Is this a node.js problem? I've also tried .hasOwnProperty('realm') and it still failed.
This is most of the code with all the functions: http://pastebin.com/DUN9VdHr

You need to parse your json into a javascript object before you can reference its properties.
You can use JSON.parse
var choiceobj = JSON.parse(choice);
if(!choiceobj.name || !choiceobj.realm || !choiceobj.region || !choiceobj.roll){
console.log(choiceobj);
console.log(choiceobj.name);
console.log(choicepbj.realm);
console.log(choiceobj.region);
console.log(choiceobj.roll);
return false;
}

The problem is that the quotes is part of the key so to access it you have to do something like:
console.log(choice['"name"']);