Develop Reference

What does this JavaScript expression mean?

I am working with react-navigation and I can't figure out the meaning of this syntax.
React.useEffect(() => {
if (route.params?.post) { <<<<<WHAT IS THIS ?
// Post updated, do something with `route.params.post`
// For example, send the post to the server
}
}, [route.params?.post]);
Does it work like obect.doesPropertyExist.subProperty or something else?
I have tried going through MDN documentation but can't find any reference to this type of syntax. I am not able to use similar syntax on a random object in my node REPL.

It is called Optional Chaining. Provided in MDN doc:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Optional_chaining
The optional chaining operator (?.) permits reading the value of a
property located deep within a chain of connected objects without
having to expressly validate that each reference in the chain is
valid. The ?. operator functions similarly to the . chaining operator,
except that instead of causing an error if a reference is nullish
(null or undefined), the expression short-circuits with a return value
of undefined. When used with function calls, it returns undefined if
the given function does not exist.
if(route.params.post) then do something.
It checks if route.params has object post

It's called Optional Chaining operator. And it's shorthand for
route.params && route.params.post

Checking for null or undefined VS handling errors VS returning bogus in JS

similar question in Java: Java: check for null or allow exception handling
related:How to deep check for "null" or "undefined" with JS?
assuming a.foo() is a well-tested function that returns either
undefined or an object with a function such as {bar:()=>log('baz')},
hence, it would seem logical to write
a.foo().bar()
which obviously won't work.
(the undefined is the result of an empty return; when no information can be delivered)
Should
1. The operation be tried,
try{a.foo().bar();}catch(e){}
2. a new variable be declared
const u = a.foo();
if(u) u.bar();
3. or foo() return {bar:()=>{}} instead of undefined?
so that
a.foo().bar();
can be written normally?
what is the industry standard? what is fastest?

As already commented, this is a personal opinion and I would also stick to it that most people would try to avoid the try-catch story...
But anyway I wanted to add that you can also simply check:
a.foo() && a.foo().bar()
in case a.foo() will return undefined, then the right side wont be executed and thus this expression return undefined.
On the other side, if a.foo() is not undefined than the right side will be executed and this expression will evaluate to the result of a.foo().bar()

Cannot determine type of undefined()

Recently I investigated a situation in which a programmer inadvertently passed undefined into addEventListener, thus:
window.addEventListener('load', undefined);
No error was thrown. It's as if JavaScript is willing to invoke undefined. But what in the world is undefined()? I have tried all sorts of things, e.g.:
console.log(undefined() === null);
console.log(typeof undefined());
but I never get anything back.
Edit added for clarity: My original question was based on a mistake, as I had not set my Developer tools to log errors to the console. The above two commands (but not the call to addEventListener) DO throw errors in a browser, as answers and comments below indicate.

It's as if JavaScript is willing to invoke undefined.
No, addEventListener is specified to ignore null, which JavaScript’s undefined is converted to. undefined is never called.
Further proof that JavaScript is not willing to invoke undefined, in a browser:
> undefined()
Uncaught TypeError: undefined is not a function
at <anonymous>:1:1

You can do something like this to find the type of passed arguments:
var myVar;
Object.prototype.toString.call(myVar);
and it will return "[object Undefined]"
same for other use cases like if myVar is a string as below:
var myVar = 'It is a string';
Object.prototype.toString.call(myVar);
it will retrun "[object String]"

It will return:
Undefined is not a function
console.log(typeof undefined());
If you run it console on a browser it will return undefined not a function. Undefined does not have any value unlike null which is an object.

null means that something exists, but has been told it has no value.
undefined means that thing has not been given a value, usually because it hasn't been declared/initialized.
In Javascript, undefined is a primitive. It's falsey, so it evaluates to False if used in conditional.
Javascript is not a strongly typed language, so there's nothing to check that a callback function is a function until it's called. In fact Javascript doesn't care how many arguments are passed or what their type is, everything's just dumped in when a function is invoked, and it's up to the function how to handle the arguments.
For example in many enumerable methods, they pass back to you (index, value, array). It doesn't matter if your function looks for these values or assigns them a temporary variable, they're still passed. Both a.forEach(function(index){}) and a.forEach(function(){}) actually have access to all 3 of the variables mentioned.

Beginner Module Pattern JavaScript - Parameters [duplicate]

I have noticed jQuery and related keynote plugins like jQuery.UI pass undefined as a parameter into anonymous functions used in their module definitions, like so:
(function($, undefined) { ... })(jQuery);
Alternatively, I have noticed other plugins recommended by jQuery and/or others do NOT pass undefined in as a parameter.
This is probably a silly question, but...
Shouldn't it always be available anyway? Why pass it in? Is there some sort of other purpose or trick going on here?

There are two reasons for that:
1) If undefined is a variable in the function scope rather than a global object property, minifiers can reduce it to a single letter thus achieving a better compression rate.
2) Before ES5*, undefined was a property of the global object without write-protection. So it could be overwritten, resulting in strange and unexpected behavior. You could do something like this:
var test = 123;
undefined = 123;
if (test === undefined){
// this will actually be true, since undefined now equals 123
}
By having an function argument undefined (the name actually does not matter) which you don't pass a parameter to, you could make sure you have a variable which really is undefined so you can test "undefinedness" against this variable.
Btw. the safest method to test for undefined is: typeof ( var ) === 'undefined'
(*) With EcmaScript 5, the global properties undefined, NaN and Infinity became readonly. So with its general adoption in all modern browsers - of course with the exception of IE 9 - overwriting those values was not possible anymore.

That is done to make sure that undefined always is undefined. In older versions of the ECMAScript spec (prior to ECMAScript 5), undefined wasn't a reserved word but a regular variable. In older browsers this would be allowed for instance:
undefined = 2; // Assign a different value to undefined
// Now this statement would be true
if (undefined == 2)
So to make sure that undefined is in fact undefined, even if some other "evil" script would have reassigned undefined with another value, you create a parameter that you call undefined, and then when you call the function, you make sure to not pass a value to that parameter - thus you can be sure that the variable undefined will always be undefined within your function.
So in the case of jQuery:
(function($, undefined) { ... })(jQuery);
They pass in jQuery and assign it to the $ variable for convenience, but then they don't pass a second value to the undefined parameter, thus undefined will be undefined.
Modern browsers
In modern browsers, undefined is a non-configurable, non-writable property per the ECMAScript 5 specification.

undefined is not a reserved word in javascript, it is simply a variable. jQuery is ensuring that if some bad developer overwrites the value of undefined in their javascript, then jQuery will ignore it and establish it's own variable, undefined, which is never passed a value (see the self-executing (jQuery) at the end) and is therefore actually undefined.

Strange && operator behaviour in node.js

My node application just spat out this error, which I don't understand:
trendData && trendData.forEach(function(trendDataItem){
^
TypeError: Object Error: HTTP Error undefined: undefined has no method 'forEach'
at /home/myapp/node/dataSource/enabled/twitterTrending.js:9:36
The && operator should be preventing any undefined values of trendData reaching the second part of the expression, but it seems not to have worked. My understanding of node.js is that my code doesn't have to concern itself with multi-threading issues, so I assume there's no chance that the value of trendData changed between the evaluation of the first part of the expression and the second.
The trendData value is passed in from a callback made by a twitter library, source code here, the line that is failing is inside the 'callback' function, from what I can see it should be the result of JSON.parse if everything worked, otherwise it will contain failure information.
Anybody have a clue what's going on?

Your understanding is correct, which means in this case trendData isn't undefined or some other falsy value, but instead is an object which doesn't have a forEach method. Based on the limited snippet provided trendData is a string containing an error message rather than the array you are expecting.