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How to turn all integers in an array into a single number? [duplicate]

How do I join this array to give me expected output in as few steps as possible?
var x = [31,31,3,1]
//expected output: x = 313131;

Use array join method.Join joins the elements of an array into a string, and returns the string. The default separator is comma (,). Here the separator should be an empty string.
var x = [31,31,3,1].join("");
EDIT: To get the result as numeric
const x = +[31,31,3,1].join("");
or
const x = Number([31,31,3,1].join(""));

Javascript join() will give you the expected output as string. If you want it as a number, do this:
var x = [31,31,3,1];
var xAsString = x.join(''); // Results in a string
var xAsNumber = Number(x.join('')); // Results in a number, you can also use +(x.join(''))

I can't think of anything other than
+Function.call.apply(String.prototype.concat, x)
or, if you insist
+''.concat.apply('', x)
In ES6:
+''.concat(...x)
Using reduce:
+x.reduce((a, b) => a + b, '');
Or if you prefer
x.reduce(Function.call.bind(String.prototype.concat), '')
Another idea is to manipulate the array as a string, always a good approach.
+String.prototype.replace.call(x, /,/g, '')
There may be other ways. Perhaps a Google search on "join array javascript" would turn up some obscure function which joins elements of an array.

Your question asks for a number, most of the answers above are going to give you a string. You want something like this.
const number = Number([31,31,3,1].join(""));

Try join() as follows
var x = [31,31,3,1]
var y = x.join('');
alert(y);

Try below.
var x = [31,31,3,1]
var teststring = x.join("");

This will work
var x = [31,31,3,1];
var result = x.join("");

Replace in javascript

I have a string something like this:
http://stackoverflow.com/questions/ask
And would like to return this part:
http://stackoverflow.com/questions/
How can I do this using pure javascript?
Thanks!

This will match and remove the last part of a string after the slash.
url = "http://stackoverflow.com/questions/ask"
base = url.replace(/[^/]*$/, "")
document.write(base)
Help from: http://www.regexr.com/

For slicing off last part:
var test = 'http://stackoverflow.com/questions/ask';
var last = test.lastIndexOf('/');
var result = test.substr(0, last+1);
document.write(result);

You can accomplish this with the .replace() method on String objects.
For example:
//Regex way
var x = "http://stackoverflow.com/questions/ask";
x = x.replace(/ask/, "");
//String way
x = x.replace('ask', "");
//x is now equal to the string "http://stackoverflow.com/questions/"
The replace method takes two parameters. The first is what to replace, which can either be a string or regex, literal or variable, and the second parameter is what to replace it with.

Incrementing a number in a string

I have the following string: 0-3-terms and I need to increment the 3 by 20 every time I click a button, also the start value might not always be 3 but I'll use it in this example..
I managed to do this using substring but it was so messy, I'd rather see if it's possible using Regex but I'm not good with Regex. So far I got here, I thought I would use the two hyphens to find the number I need to increment.
var str = '0-3-terms';
var patt = /0-[0-9]+-/;
var match = str.match(patt)[0];
//output ["0-3-"]
How can I increment the number 3 by 20 and insert it back in to the str, so I get:
0-23-terms, 0-43-terms, 0-63-terms etc.

You're doing a replacement. So use .replace.
var str = '0-3-terms';
var patt = /-(\d+)-/;
var result = str.replace(patt,function(_,n) {return "-"+(+n+20)+"-";});

Another option is to use .split instead of regex, if you prefer. That would look like this:
var str = '0-3-terms';
var split = str.split('-');
split[1] = +split[1] + 20;
var result = split.join('-');
alert(result);

I don't understand why you are using regex. Simply store the value and create string when the button is called..
//first value
var value = 3;
var str = '0-3-terms';
//after clicking the button
value = value+20;
str = "0-" + value + "-terms"

get index value from string representing array jquery

given string is
var strg = "RoomItemsQuantity[items][1][itemId]"
I'm having the above string value.How can i get itemId from the string.
var index = strg.lastIndexOf("[");
var fnstrg = strg.substr(index+1);
var fnstrg = fnstrg.replace("]","");
I've done like this is there any easiest way to do this?
thanks,

This code...
var arr = "RoomItemsQuantity[items][1][itemId]".replace(/]/g, "").split("[")
Will result in an array like this:
["RoomItemsQuantity", "items", "1", "itemId"]
Then...
arr[arr.length - 1]
Will give you what you want.

If the above pattern is fixed, then you could use the following trick :
var myString = "RoomItemsQuantity[items][1][itemId]";
var myItemId = (myString+"[").split("][")[2]

You can use a regular expression like:
('RoomItemsQuantity[items][1][itemId]'.match(/([^\[]+)\]$/) || [])[1] // itemId
You can also use lookahead or lookbehind but support may not be available.

how to get numbers from a string in javascript?

I have
"id": 1468306
inside of a string, how can I use regular expression to get the number 1468306 for it?

You can use this regex:
/: (\d+)/
as in:
s = '"id": 1468306';
r = /: (\d+)/;
console.log(r.exec(s)[1]);
Output:
1468306

you can use parseInt() method in javascript as follows:
var str = parseInt(id);

Following code may help you:
var input = '"id": 1468306';
var matches = input.match(/"id": (\d+)/);
var id = matches[1];
The id get the required number.

JSON.parse("{" + yourString + "}").id
Will be your number if you have that in a String.

Fiddle: http://jsfiddle.net/SfeMh/
var regEx = /\d+/g;
var str = '"id": 1468306';
var numbers = str.match(regEx);
alert(numbers); // returns 1468306
It looks like you're trying to parse a JSON String. Try this way as already mentioned:
var parsedObj = JSON.parse(myJSONString);
alert(parsedObj.id); // returns 1468306

This will match in this cases
id : 156454;
id :156454;
id:156454;
/id\s?[:]\s?[0-9]+/g.match(stringhere)

Alright, my JSON answer still stands, use it if that's your full string you're giving us in the question. But if you really want a regex, here's one that will search for "id" and then find the number after.
parseInt(yourString.match(/("id"\s?:\s?)(\d+)/)[2])
Fiddle: http://jsfiddle.net/tS9M4/