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How to use Regex to see if String matches another string partially (or completely)?

I am making a trivia system, and the variable triviaA is changed to the updated Answer every time there is a new question. I was wondering how I could use Regex to make it so that if triviaA = 'eagle' then if someone submitted the answer as eagle but with a small spelling mistake, the if statement would still return both the triviaA answer and the players answer return true.
I'm new to Regex, please excuse my mistakes.

You can dynamically build a regular expression that matches all possible "errors" (wrong/missing/extra letter) and apply it to the source:
function fuzzyContains(word, str) {
let r = [];
for (let i = 0; i < word.length; i++) {
// wrong letter
r.push(word.slice(0, i) + '[a-z]' + word.slice(i + 1));
// missing letter
r.push(word.slice(0, i) + '' + word.slice(i + 1));
// extra letter
r.push(word.slice(0, i) + '[a-z]' + word.slice(i));
}
return new RegExp(r.join('|')).test(str)
}
console.log(fuzzyContains('eagle', 'fly, ewgle, fly'))
console.log(fuzzyContains('eagle', 'fly, eagl, fly'))
console.log(fuzzyContains('eagle', 'fly, eaggle, fly'))
console.log(fuzzyContains('eagle', 'fly, eagly, fly'))
See https://norvig.com/spell-correct.html for other interesting ideas.

This would actually be quite difficult to do with RegEx. However, there's a concept in computing called "edit distance", which is a measure of how "similar" two strings are, and there are known algorithms to calculate that. Which means there are Node packages to calculate it :)
For instance, there's the aptly named Node package edit-distance: https://www.npmjs.com/package/edit-distance (Note: edit distance is sometimes called Levenshtein edit distance, named after the man who first studied it.)
I'll give you an example using that package; other packages may work somewhat differently to calculate the same thing.
There are three types of changes between two strings: an insertion, where a character is added in one that's not in the other; a deletion, where a character is removed in one that's there in the other; and an update/substitution, where a letter is changed between the strings. Using the edit-distance package, you define a function that assigns a cost to each of these types of changes. Then you call the package's levenshtein method, passing it the two strings and the three functions, and it returns an object with a distance property that is the sum score.
Assuming your cost functions return non-negative values, a score of 0 means the two strings are identical, and higher numbers mean they're more different. So you can use this to compare the entered value with the correct string and, if the result is lower than a certain threshold, accept it as "correct excluding typos".

Data changes after assigning to another variable

I work with Discord.js User object and Mongoose Schema. But the problem doesn't seems to be part of those.
var Message = require('../app/models/message'); //Mongoose Schema
...
var newMessage = new Message();
...
//taggedUser is an object containing all the info about user. id property contains user id which is number.
const taggedUser = message.mentions.users.first();
newMessage.message.to = taggedUser.id;
console.log(taggedUser.id);
console.log(newMessage.message.to);
The code above should assign user ID to Schema. Everything works, but...
442090269928849410
442090269928849400
Last 2 characters aren't the same among these variables now. How is this even possible? The = changed the actual data inside the variable?
In case it is Mongoose here is how Schema looks like:
var msgSchema = mongoose.Schema({
message : {
from : Number,
to : Number,
content : String,
time : Date
}
});
Edit:
If I change
to : Number,
to string:
to : String,
It works properly. I still need the answer on why does this work incorrectly with number. Right above the problematic line I have another id which works perfectly fine:
newMessage.message.from = msg.author.id;
I have already tried to parse taggedUser.id to integer or creating Number() object but that didn't help. So every time I turn taggedUser.id into a Number or parse it to int it changes to the slightly different number.
I don't know what to think. How can data change during the assignment?
If there is not enough data provided in the question please ask me and I'll add everything needed. I can't imagine what might be causing this bug.

9007199254740992 - Highest safe number in JS
442090269928849410 - Your integer (id)
The reason of that small variation is the 'Max precision' JavaScript can work with.
When you tried to use the id as a number it was affected by this and it changed because JavaScript can't be that precise.
If you see both numbers at the beginning of this answer you can see that they are separated by 2 characters, that is why only the 2 last character changed.
Basically your integer was affected by the max precision JS numbers can have.
Reference: https://developer.mozilla.org/es/docs/Web/JavaScript/Referencia/Objetos_globales/Number/MAX_SAFE_INTEGER

You might just be seeing an artifact of console.log running asynchronously. Try this:
console.log('' + taggedUser.id);
console.log('' + newMessage.message.to);
...and see if that makes any difference.

What is the ??! operator in Javascript?

when I'm looking for some sites Javascript code, I see this
function hrefLeftMenu() {
var x = true;
for (i in info) {
$(".leftmenu ul").append("<li" + (x ? " class='active'" : "") + " onclick='openAnInfo(\"" + i + "\", this);'> - " + info[i].title + "</li>");
x = x??!x;
}
openAnInfo("0", ".lelelesakineyy");
}
What it does in javascript? Why the coder's used this operator?
Thanks.

What it does in javascript?
It throws a syntax error.
> x = x??!x;
SyntaxError: Unexpected token ?
Why the coder's used this operator?
Taking a reasonable guess (beyond "they made a mistake") would need more context. Saying for sure would require mind reading :)

In JavaScript this is not valid code. However, the sequence ??! exists (existed?) in C as a trigraph, representing |. Maybe that's not JavaScript code, or it was poorly-ported from ancient C code. But even then, x = x | x can hardly be called a useful statement.
EDIT: With a bit context in the question now, that speculation is likely wrong. A friend suggested that maybe the sequence x?? was a typo and subsequent attempt to correct it where backspace was turned into a character by some intermediate mangling (not uncommon when typing in terminals or via SSH) and that the line in question was supposed to be x = !x.

I think it is a mistake. They're generating a menu and x is used to set an item as active, and it looks like they want to default to selecting the first item. They want x to be true the first time around, and then false for the rest. It was probably supposed to be something like
x = x?!x:x; // If first time through, then set x = false for the rest
aka
x = false; // Set x = false for the rest
but confusion/muddy thinking led to over-complification.

Was this a mistake?
Did you mean this?
x= x?x:!x;

Weird JavaScript Code

for (var i=a.length-1;i>0;i--) {
if (i!=a.indexOf(a.charAt(i))) {
a=a.substring(0,i)+a.substring(i+1);
}
}
I found this in a web app I'm auditing, it just baffles me why it's there.
I can't seem to see a case where i!=a.indexOf(a.charAt(i)) would be false.
The value the pass to it is:
a = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
There is no comment either //sigh

This would be true for repeated characters, since indexOf finds the first index of a string, and you're searching from the end. Example:
var a = "xyzxyz";
On first iteration, i === 4, a.charAt(4) === "x", and a.indexOf("x") === 0. So 4 !== 0.
It then sets a = a.substring(0, 4) + a.substring(5). Recalling at substring is inclusive in the first index but exclusive in the last index, that means in this case a = "xyz" + "yz", so we have removed the duplicate "x" from the string.
Since the loop traverses backward, this will continue to work even for characters repeated more than once; you can see that the portion a.substring(i + 1) will always have been covered by the algorithm already, i.e. not contain any duplicates.
As always when encountering this type of thing, applying the extract method refactoring would be a great way to make the code clearer. (Even better than commenting it!) So if you just pulled this out into a method, the code could become a = removeDuplicateChars(a), and everyone is much happier.

Equality testing in Javascript

just trying to test for equality in this piece of code, but getting a fail.
<input type="text" name="dave_blah"/>
<input type="text" name="dave_bleh"/>
I then access the name values of each of these inputs and assign them to two variables, name1 and name2. I then extract the first part of the name,delimited by a "_".
var oldName = name1.name.split('_',1);//dave
var thisName= name2.name.split('_',1);//dave
alert(oldName);
alert(thisName);
if(oldName !== thisName){//if "dave" is not equal to "dave"
alert("name difference = "+ oldName + " " + thisName);
}
Yet, when running this code, the message alerts regardless (I've tried != too). In principle, the alert shouldn't execute. It's quite late in the evening, so it's probably obvious, but can someone point this noob in the right direction? If I remove the not operator from the if statement - the function works as desired.

thisName and oldName are both arrays, do something like this:
var oldName = name1.name.split('_',1)[0]; //dave
var thisName= name2.name.split('_',1)[0]; //dave
And I think it should work.

OK. I've sussed the problem. The comparison was seeing the participants in the test both as Objects, as opposed to the string value of the contents. So, I solved it by casting the results to a string.