You can use the following.

window.open(
  'https://google.com',
  '_blank' // <- This is what makes it open in a new window.
);

in HTML

 <button class="btn btn-success" onclick=" window.open('http://google.com','_blank')"> Google</button>

plunkr

Since this is javascript you need to use like this:

<form>
   <input TYPE="button" VALUE="Home Page"
    onclick="window.open('http://www.wherever.com');"> 
</form>

Use this:

<input type="button" value="button name" onclick="window.open('http://www.website.com/page')" />

Worked for me and it will open an actual new 'popup' window rather than a new full browser or tab. You can also add variables to it to stop it from showing specific browser traits as follows:

onclick="window.open(this.href,'popUpWindow','height=400,width=600,left=10,top=10,,scrollbars=yes,menubar=no'); return false;"

In addition to the other answers, add "_blank" to open in a new tab if you intend on using a JavaScript function.

function AsDownload() {
  //window.open(pathString, target);
  window.open("https://alexanderhawking.itch.io/asteroid-racer", "_blank");
};

<button id="AsDownload" onclick="AsDownload()">Download</button>

Set the target attribute of the link to _blank:

<a href="#" target="_blank" rel="noopener noreferrer">Link</a>

For other examples, see here: http://www.w3schools.com/tags/att_a_target.asp

Note

I previously suggested blank instead of _blank because, if used, it'll open a new tab and then use the same tab if the link is clicked again. However, this is only because, as GolezTrol pointed out, it refers to the name a of a frame/window, which would be set and used when the link is pressed again to open it in the same tab.

Security Consideration!

The rel="noopener noreferrer" is to prevent the newly opened tab from being able to modify the original tab maliciously. For more information about this vulnerability read the following articles:

• The target="_blank" vulnerability by example
• External Links using target='_blank'