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Make string comma delimited in JavaScript

I have a string:
var myStr = 'This is a test';
I would like to make it comma delimited via JavaScript, like so:
var myNewStr = 'This, is, a, test';
What is the best way to accomplish this via JavaScript?
I know I can trim the commas like so:
var myStr = myNewStr.split(',');
And that would give me this:
myStr = 'This is a test';
But I am wanting to do the opposite of course, by adding commas.

You could just replace with regex - it is shorter and no need to split.
var myNewStr = myStr.replace(/ /g,', ');
In case you could also face the string with leading / trailing spaces, just trim it beforehand.
var myNewStr = myStr.trim().replace(/ /g,', ');

Try this - var myNewStr = myStr.split(' ').join(', ')

You could use a regular expression with a positive lookahead and replace then with a comma.
console.log('This is a test'.replace(/(?= .)/g, ','));
console.log('This is a test '.replace(/(?= .)/g, ','));

Use String.replace:
var myStr = 'This is a test';
var myNewStr=myStr.replace(/ /g,', ');

You could use the replace function to replace the spaces with commas:
var myStr = myNewStr.replace(' ', ',');
You could also use:
var myStr = myNewStr.split(' ');

Here is a way to do it without using the standard methods.
var myStr = "THIS IS A TEST";
before.innerHTML = myStr;
var parts = [];
var buffer = '';
for(var i = 0; i < myStr.length; i++) {
if(myStr[i] == ' ') {
parts.push(buffer);
buffer = '';
continue;
} else {
buffer += myStr[i];
}
}
parts.push(buffer)
var myNewStr = parts.join(', ');
after.innerHTML = myNewStr;
<div><b>Before</b></div>
<div id="before"></div>
<div><b>After</b></div>
<div id="after"></div>

The solution using String.trim and String.replace functions:
var myStr = ' This is a test ',
myNewStr = myStr.trim().replace(/([^\s]+\b)(?!$)/g, "$&,");
// $& - Inserts the matched substring.
console.log(myNewStr); // "This, is, a, test"

Javascript regex get string before second /

var str = '#/promotionalMailer/test1';
output should be ==> #/promotionalMailer
I want the string before the second slash '/'
I have tried this so far:
var str = '#/promotionalMailer/test1';
var match = str.match(/([^\/]*\/){2}/)[0];
alert(match);
But it comes with the second slash.

try split, slice and join
var str = '#/promotionalMailer/test1';
console.log( str.split("/").slice(0,2).join("/"));

For example,
var str = '#/promotionalMailer/test1/foo/bar/baz';
result = str.split('/').slice(0, 2).join('/')
document.write('<pre>'+JSON.stringify(result,0,3));
If you want regexes, then
var str = '#/promotionalMailer/test1/foo/bar/baz';
result = str.match(/[^\/]*\/[^\/]*/)[0]
document.write('<pre>'+JSON.stringify(result,0,3));

How to cut third symbol from string and alert it without third symbol

How can i correctly cut out letter "v" and alert str without "v"
var str = "Javascript";
var cut = str.substring(2,3);

var str = "Javascript";
var cut = str.substring(0,2) + str.substring(3);
alert(cut);

You're using the right tool (String#substring). You need two substrings that you put back together, the "Ja" and the "ascript". So:
var str = "Javascript";
var cut = str.substring(0, 2) + str.substring(3);
alert(cut);
Another option would be String#replace, which will replace the first occurrence of what you give it unless you tell it to do it globally with a regex and the g flag (which we won't, because we just want to remove that one v):
var str = "Javascript";
var cut = str.replace("v", "");
alert(cut);
Just for fun, there is another way, but it's a bit silly: You can split the string into an array of single-character strings, remove the third entry from the array, and then join it back together:
var str = "Javascript";
var cut = str.split("").filter(function(_, index) {
return index != 2;
}).join("");
alert(cut);
or
var str = "Javascript";
var cut = str.split("");
cut.splice(2, 1); // Delete 1 entry at index 2
cut = cut.join("");
alert(cut);
...but again, that's a bit silly. :-)

use replace method
var str = "Javascript";
str = str.replace("v", "");
alert(str);

How to get particular string from one big string ?

I have string
var str = "Ahora MXN$1,709.05" and wanted to get only
"MXN$1,709.05" from this.
Can someone please help me?

You can use substring or replace. With replace you are going to replace something with nothing.
replace
var str = 'Ahora MXN$1,709.05';
var sub = 'Ahora ';
var res = str.replace(sub,'');
substring
var str = 'Ahora MXN$1,709.05';
var sub = 'Ahora ';
var res = str.substring(sub.length);
JsFiddle

You can use either substring or Regex
Using substring
var str = "Ahora MXN$1,709.05";
var result = str.substring('Ahora '.length);
console.log(result);
Using Regex
var str = "Ahora MXN$1,709.05";
var myRegexp = /Ahora\s(.*?)(?:\s|$)/g;
var match = myRegexp.exec(str);
console.log(match[1]);

Replace each leading and trailing whitespace with underscore using regex in javascript

var str = ' Some string ';
var output = str.replace(/^\s|\s(?=\s*$)/g , '_');
The output should look like this
'___Some string____'
This code works fine for the trailing whitespaces but all the leading whitespaces are replaced with just one underscore.
The working php regex for this is: /\G\s|\s(?=\s*$)/

Not pretty, but gets the job done
var str = " Some string ";
var newStr = str.replace(/(^(\s+)|(\s+)$)/g,function(spaces){ return spaces.replace(/\s/g,"_");});

This works but I don't like it:
var str = " some string ";
var result = str.replace(/^\s+|\s+$/g, function(m) {
return '________________________________________'.substring(0, m.length);
});
Or more flexibly:
var str = " some string ";
var result = str.replace(/^\s+|\s+$/g, function(m) {
var rv = '_',
n;
for (n = m.length - 1; n > 0; --n) {
rv += '_';
}
return rv;
});
That can be refined, of course.
But I like epascarello's answer better.

another reg ex
var str = " asdf "
str.replace(/^[ ]+|[ ]+$/g,function(spc){return spc.replace(/[ ]/g,"_")})
//"__asdf_______"

With String.prototype.repeat() the accepted answer can now be improved to
function replaceLeadingAndTrailingWhitespace(text, replace) {
return text.replace(/^\s+|\s+$/g, (spaces) => replace.repeat(spaces.length));
}