Using a Library

If you are doing a lot of date work, you may want to look into JavaScript date libraries like Datejs or Moment.js. For example, with Moment.js, this is simply:

var newDateObj = moment(oldDateObj).add(30, 'm').toDate();

Vanilla Javascript

This is like chaos's answer, but in one line:

var newDateObj = new Date(oldDateObj.getTime() + diff*60000);

Where diff is the difference in minutes you want from oldDateObj's time. It can even be negative.

Or as a reusable function, if you need to do this in multiple places:

function addMinutes(date, minutes) {
    return new Date(date.getTime() + minutes*60000);
}

^{And just in case this is not obvious, the reason we multiply minutes by 60000 is to convert minutes to milliseconds.}

Be Careful with Vanilla Javascript. Dates Are Hard!

You may think you can add 24 hours to a date to get tomorrow's date, right? Wrong!

addMinutes(myDate, 60*24); //DO NOT DO THIS

It turns out, if the user observes daylight saving time, a day is not necessarily 24 hours long. There is one day a year that is only 23 hours long, and one day a year that is 25 hours long. For example, in most of the United States and Canada, 24 hours after midnight, Nov 2, 2014, is still Nov 2:

const NOV = 10; //because JS months are off by one...
addMinutes(new Date(2014, NOV, 2), 60*24); //In USA, prints 11pm on Nov 2, not 12am Nov 3!

This is why using one of the afore-mentioned libraries is a safer bet if you have to do a lot of work with this.

Below is a more generic version of this function that I wrote. I'd still recommend using a library, but that may be overkill/impossible for your project. The syntax is modeled after MySQL DATE_ADD function.

/**
 * Adds time to a date. Modelled after MySQL DATE_ADD function.
 * Example: dateAdd(new Date(), 'minute', 30)  //returns 30 minutes from now.
 * 
 * 
 * @param date  Date to start with
 * @param interval  One of: year, quarter, month, week, day, hour, minute, second
 * @param units  Number of units of the given interval to add.
 */
function dateAdd(date, interval, units) {
  if(!(date instanceof Date))
    return undefined;
  var ret = new Date(date); //don't change original date
  var checkRollover = function() { if(ret.getDate() != date.getDate()) ret.setDate(0);};
  switch(String(interval).toLowerCase()) {
    case 'year'   :  ret.setFullYear(ret.getFullYear() + units); checkRollover();  break;
    case 'quarter':  ret.setMonth(ret.getMonth() + 3*units); checkRollover();  break;
    case 'month'  :  ret.setMonth(ret.getMonth() + units); checkRollover();  break;
    case 'week'   :  ret.setDate(ret.getDate() + 7*units);  break;
    case 'day'    :  ret.setDate(ret.getDate() + units);  break;
    case 'hour'   :  ret.setTime(ret.getTime() + units*3600000);  break;
    case 'minute' :  ret.setTime(ret.getTime() + units*60000);  break;
    case 'second' :  ret.setTime(ret.getTime() + units*1000);  break;
    default       :  ret = undefined;  break;
  }
  return ret;
}

Working jsFiddle demo.

var d1 = new Date (),
    d2 = new Date ( d1 );
d2.setMinutes ( d1.getMinutes() + 30 );
alert ( d2 );

Maybe something like this?

var d = new Date();
var v = new Date();
v.setMinutes(d.getMinutes()+30);

console.log(v)

I always create 7 functions, to work with date in JS:
addSeconds, addMinutes, addHours, addDays, addWeeks, addMonths, addYears.

You can see an example here:

How to use:

var now = new Date();
console.log(now.addMinutes(30));
console.log(now.addWeeks(3));

These are the functions:

Date.prototype.addSeconds = function(seconds) {
  this.setSeconds(this.getSeconds() + seconds);
  return this;
};

Date.prototype.addMinutes = function(minutes) {
  this.setMinutes(this.getMinutes() + minutes);
  return this;
};

Date.prototype.addHours = function(hours) {
  this.setHours(this.getHours() + hours);
  return this;
};

Date.prototype.addDays = function(days) {
  this.setDate(this.getDate() + days);
  return this;
};

Date.prototype.addWeeks = function(weeks) {
  this.addDays(weeks*7);
  return this;
};

Date.prototype.addMonths = function (months) {
  var dt = this.getDate();
  this.setMonth(this.getMonth() + months);
  var currDt = this.getDate();
  if (dt !== currDt) {  
    this.addDays(-currDt);
  }
  return this;
};

Date.prototype.addYears = function(years) {
  var dt = this.getDate();
  this.setFullYear(this.getFullYear() + years);
  var currDt = this.getDate();
  if (dt !== currDt) {  
    this.addDays(-currDt);
  }
  return this;
};

Stop using Moment.js

As recommended by other great answers, in most cases it's best to use a library when dealing dates. However, it's important to know that as of September 2020 Moment.js is considered legacy and should no longer be used in new projects.

Quoting Moment's statement in their official docs:

We would like to discourage Moment from being used in new projects going forward. [...] We now generally consider Moment to be a legacy project in maintenance mode. It is not dead, but it is indeed done.

Modern libraries

Below are alternatives recommended by Moment.

Luxon

Luxon can be thought of as the evolution of Moment. It is authored by Isaac Cambron, a long-time contributor to Moment. Please read Why does Luxon exist? and the For Moment users pages in the Luxon documentation.

• Locales: Intl provided
• Time Zones: Intl provided

import {DateTime} from 'luxon'

function addMinutes(date, minutes) {
    return DateTime.fromJSDate(date).plus({minutes}).toJSDate()
}