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Replacement of "-' with "_" using a loop in javascript

I have applied the same method to replace "-" with "_" in c++ and it is working properly but in javascript, it is not replacing at all.
function replace(str)
{
for(var i=0;i<str.length;i++)
{
if(str.charAt(i)=="-")
{
str.charAt(i) = "_";
}
}
return str;
}

It's simple enough in javascript, it's not really worth making a new function:
function replace(str){
return str.replace(/-/g, '_') // uses regular expression with 'g' flag for 'global'
}
console.log(replace("hello-this-is-a-string"))
This does not alter the original string, however, because strings in javascript are immutable.
If you are dead set on avoiding the builtin(maybe you want to do more complex processing), reduce() can be useful:
function replace(str){
return [...str].reduce((s, letter) => s += letter == '-' ? '_' : letter , '')
}
console.log(replace("hello-this-is-a-string"))

This is yet another case of "make sure you read the error message". In the case of
str.charAt(i) = "_";
the correct description of what happens is not "it is not replacing at all", as you would have it; it is "it generates a run-time JavaScript error", which in Chrome is worded as
Uncaught ReferenceError: Invalid left-hand side in assignment
In Firefox, it is
ReferenceError: invalid assignment left-hand side
That should have given you the clue you needed to track down your problem.
I repeat: read error messages closely. Then read them again, and again. In the great majority of cases, they tell you exactly what the problem is (if you only take the time to try to understand them), or at least point you in right direction.
Of course, reading the error message assumes you know how to view the error message. Do you? In most browsers, a development tools window is available--often via F12--which contains a "console", displaying error messages. If you don't know about devtools, or the console, then make sure you understand them before you write a single line of code.
Or, you could have simply searched on Stack Overflow, since (almost) all questions have already been asked there, especially those from newcomers. For example, I searched in Google for
can i use charat in javascript to replace a character in a string?
and the first result was How do I replace a character at a particular index in JavaScript?, which has over 400 upvotes, as does the first and accepted answer, which reads:
In JavaScript, strings are immutable, which means the best you can do is create a new string with the changed content, and assign the variable to point to it.
As you learn to program, or learn a new languages, you will inevitably run into things you don't know, or things that confuse you. What to do? Posting to Stack Overflow is almost always the worst alternative. After all, as you know, it's not a tutorial site, and it's not a help site, and it's not a forum. It's a Q&A site for interesting programming questions.
In the best case, you'll get snarky comments and answers which will ruin your day; in the worst case, you'll get down-voted, and close-voted, which is not just embarrassing, but may actually prevent you from asking questions in the future. Since you want to make sure you are able to ask questions when you really need to, you are best off taking much more time doing research, including Google and SO searches, on simple beginner questions before posting. Or find a forum which is designed to help new folks. Or ask the person next to you if there is one. Or run through one or more tutorials.
But why write it yourself at all?
However, unless you are working on this problem as a way of teaching yourself JavaScript, as a kind of training exercise, there is no reason to write it at all. It has already been written hundreds, or probably thousands, of times in the history of computing. And the overwhelming majority of those implementations are going to be better, faster, cleaner, less buggy, and more featureful than whatever you will write. So your job as a "programmer" is not to write something that converts dashes to underscores; it's to find and use something that does.
As the wise man said, today we don't write algorithms any more; we string together API calls. Our job is to find, and understand, the APIs to call.
Finding the API is not at all hard with Google. In this case, it could be helpful if you knew that strings with underscores are sometimes called "snake-cased", but even without knowing that you can find something on the first page of Google results with a query such as "javascript convert string to use underscores library".
The very first result, and the one you should take a look at, is underscore.string, a collection of string utilities written in the spirit of the versatile "underscore" library, and designed to be used with it. It provides an API called underscored. In addition to dealing with "dasherized" input (your case), it also handles other string/identifier formats such as "camelCase".
Even if you don't want to import this particular library and use it (which you probably should), you would be much better off stealing or cloning its code, which in simplified form is:
str
.replace(/([a-z\d])([A-Z]+)/g, '$1_$2')
.replace(/[-\s]+/g, '_')
This is not as complicated as it looks. The first part is to deal with camelCase input, so that "catsAndDogs" becomes "cats-and-dogs". the second line is what handles the dashes and spaces). Look closely--it replaces runs of multiple dashes with a single underscore. That could easily be something that you want to do too, depending on who is doing what with the transformed string downstream. That's a perfect example of something that someone else writing a professional-level library for this task has thought of that you might not when writing your home-grown solution.
Note that this well-regarded, finely-turned library does not use the split/join trick to do global replaces in strings, as another answer suggests. That approach went out of use almost a decade ago.
Besides saving yourself the trouble of writing it yourself, and ending up with better code, if you take time time to understand what it's doing, you will also end up knowing more about regular expressions.

You can easily replace complete string using .split() and .join().
function replace(str){
return str.split("-").join("_");
}
console.log(replace("hello-this-is-a-string"))

VBScript integers followed by #

I've two dimensional array full of integers that I need to rewrite in Javascript from VBScript, and it looks like:
a(1, 1) = 0.03435435:
a(1, 2) = 0#:
What is the 0#?

As the comments say, that looks very odd, and syntactically incorrect. Other VB languages support suffixes for types, where # on a variable or literal means that it's a double, but VBScript only has Variant variables, so it's a syntax error in VBScript.
All the lines ending with a colon is also suspect. While that's legal syntax (as the colon separates statements on a single line, and one can have empty statements), it has no effect at the end of lines when there's no statement following. I have a hunch that this code was ported to or written in VBScript by somebody who was not entirely familiar with it.
If this code is actually running somewhere and not erroring, is it possible that On Error Resume Next is on? If so, and the line is setting a value to 0, since the default value of variables is Empty (and thus converted to 0 when used in a math context), it may just be that the line has never worked, the error got ignored, and yet the program kept "working" anyway by pure coincidence. This is just some complete speculation on my part, though.
If you're trying to port over what the original code it doing exactly, and you can modify and run the original, it may be worthwhile to print out the contents of that array after it's initialized, just so you can confirm that it indeed has the values that it looks like it should. It may not be the same as the values the original author intended, though, if there are other problems with other values.

JavaScript "\8" literal

I am writing JavaScript to Python translator and "\8" and "\9" are causing me lot's of problems. According to the documentation something like "\8" or "\9" is illegal since they are not valid octal escapes. Esprima parser throws exception on such literal. However JS engines they seem to allow them and they evaluate to "8" and "9" respectively.
Therefore:
/\8/.exec("\8")
RegExp('\\8').exec('\8')
/\8/.exec("8")
RegExp('\\8').exec('8')
Should all return a match since /\8/ should be the same as /8/. However the results are inconsistent across JS engines and some return a match while others don't (for example Safari's).
What's the reason for all these differences? And what is the right way - how to handle other cases involving these literals?

You're right that the spec does not allow for it but no one ever said that JS engines are perfect.
The "right" way to handle those cases is to report them as a syntax error, given that this isn't valid in JS nor Python*.
*As far as I know. I don't write a lot of Python but a quick Googling seems to indicate it isn't.

Accurately unescape HTML entities in javascript

In javascript, I need to take a string and HTML un-escape it.
This question over here asks the same question, and the most popular answer involves populating a temporary div.
I've used this as well, but I think I've found a bug.
Simple example, correct behavior
If you have this string: Cats>Dogs
Unescaped, it should be: Cats>Dogs
Malformed example, wrong behavior
If you remove the semicolon and use this instead:Cats&gtDogs
You will get this as a result: Cats>Dogs
Isn't that wrong?
This struck me as odd. From what I understand, an escaped string requires the presence of a terminating semicolon, otherwise it's not escaped. After all, what if I had a store called guitars&amps? For all we know, this company exists but gets no business because it causes null reference exceptions everywhere it has records.
Any ideas on how I could perform escaping while knowingly avoiding escaping when the semicolon is missing? Currently, all I can think to do is perform the unescaping myself.
(The WYSIWYG preview in StackOverflow exhibits a similar unusual behavior, by the way. Try entering &ampgt;, this renders as >!)

Isn't that wrong?
Successful HTML parsers are tolerant. This is one of the things distinguishing them from, say, XML parsers. They don't necessarily stick to strict rules about markup, for the simple reason that there's a lot of incorrect markup out there. So they try to figure out what the markup is meant to represent. &gtDogs is more likely to mean >Dogs than &gtDogs, so that's what the parser goes with.

How Do I Sanitize JS eval Input?

a="79 * 2245 + (79 * 2 - 7)";
b="";
c=["1","2","3","4","5","6","7","8","9","0","+","-","/","*"];
for (i=1;i<a.length;i++){
for (ii=1;i<c.length;i++){
b=(a.substring(0,i))+(c[ii])+(a.substring(i+1,a.length));
alert(eval(b.replace(" ","")));
}
}
I need to find out how to make it so that when I use eval, I know that the input will not stop the script, and if it would normally crash the script to just ignore it. I understand that eval is not a good function to use, but I want a quick and simple method by which I can solve this. The above code tries to output all of the answers with all of the possible replacements for any digit, sign or space in the above. i represents the distance through which it has gone in the string and ii represents the symbol that it is currently checking. a is the original problem and b is the modified problem.

Try catching the exception eval might throw, like this:
try{
alert(eval(b.replace(" ","")));
} catch (e){
//alert(e);
}

You can check for a few special cases and avoid some behaviors with regex or the like, but there is definitely no way to 'if it would normally crash just ignore it'
That is akin to the halting problem, as mellamokb refers to. And theres no way to know ipositively f a script runs to completion besides running it.
One should be very careful to vet any strings that go to eval, and keep user input out of them as much as possibl except for real simple and verifiable things like an integer value. If you can find a way around eval altogether than all the better.
For the calculation example you show its probably best to parse it properly into tokens and go from there rather than evaluate in string form.
PS - if you really want to check out these one-off's to the expression in a, it is a somewhat interesting use of eval eespite its faults. cam you explain why you are trimming the whitespace imediately before evaluation? i dont believe i can think of a situation where it effects the results. for (at least most) valid expressions it makes no difference, and while it might alter some of the invalid cases i cant think of a case where it does so meaningfully