Javascript and backslashes replace

Got stumped by this for ages and all the answers kept insisting that the source string needs to already have escaped backslashes in it ... which isn't always the case.

Do this ..

var replaced = str.replace(String.fromCharCode(92),String.fromCharCode(92,92));

The string doesn't contain a backslash, it contains the \s escape sequence.

var str = "This is my \\string";

And if you want a regular expression, you should have a regular expression, not a string.

var replaced = str.replace(/\\/, "\\\\");

The problem is that the \ in your first line isn't even recognized. It thinks the backslash is going to mark an escape sequence, but \s isn't an escape character, so it's ignored. Your var str is interpreted as just "This is my string". Try str.indexOf("\\") - you'll find it's -1, since there is no backslash at all. If you control the content of str, do what David says - add another \ to escape the first.

var a = String.raw`This is my \string`.replace(/\\/g,"\\\\");
console.log(a);

Result:

This is my \\string

In case you have multiple instances or the backslash:

str.split(String.fromCharCode(92)).join(String.fromCharCode(92,92))

Use this

str.replace(/(\s)/g,function($0){return $0==' '?' ':'\\s'})

or

str.replace(/ /g,'something').replace(/\s/g,'\\s').replace(/something/g,' ');

'something' it may be a combination of characters that is not in string

var str=' \s';  
  str.replace(/\s/g,'\\s'); 
// return '\\s\\s'   
  str.replace(/ /g,'SpAcE').replace(/\s/g,'\\s').replace(/SpAcE/g,' ');
// return ' \\s' 

I think the confusion is coming from how a string is displayed in the browser console. (I just checked chrome)

let myString = "This is my \string";

Here myString is actually doesn't include any backslash char. It includes \s sequence which resolves to s.

So If you display your string in the console:

> myString => 
"This is my string"

> console.log(myString) 
This is my string

The following string though has only 1 backslash in it:

let myString = "This is my \\string";


//string representation includes the escape char as well. So we see double backslashes.  
> myString => 
"This is my \\string" 

//When you print it, the escape char is not there. 
> console.log(myString) 
This is my \string 

So finally if you want to replace backslash char with 2 backslashes you can do this:

let myNewStringWith2Backslashes= myString.replace("\\","\\\\")

If use case is to replace some values in the toString of a function, and convert the string back to a valid function.

var exFnStr1 = exFn.toString();

var exFnStr = "";
var quoteStarted = false;
for(i = 0; i < exFnStr1.length; i++) {
    var iChar = exFnStr1.charAt(i);
    var oChar = exFnStr1.charAt(i);
    var currentCharCode = exFnStr1.charCodeAt(i);
    if(quoteStarted) {
        if(currentCharCode === 9) oChar = "tabChar";
        if(currentCharCode === 10) oChar = "newlineChar";
    }
    //console.log(iChar+"->"+currentCharCode+"->"+oChar)
    exFnStr += oChar;

    if(currentCharCode === 34) {
        if(quoteStarted) quoteStarted = false;
        else quoteStarted = true;
    }
}
console.log(exFnStr);

//TODO - replace values in the string

exFn = new Function('return ' + exFnStr)();

I haven't tried this, but the following should work

var replaced = str.replace((new RegExp("\s"),"\\s");

Essentially you don't want to replace "\", you want to replace the character represented by the "\s" escape sequence.

Unfortunately you're going to need to do this for every letter of the alphabet, every number, symbol, etc in order to cover all bases