How to trim a string to N chars in Javascript?

How can I, using Javascript, make a function that will trim string passed as argument, to a specified length, also passed as argument. For example:

var string = "this is a string";
var length = 6;
var trimmedString = trimFunction(length, string);

// trimmedString should be:
// "this is"

Anyone got ideas? I've heard something about using substring, but didn't quite understand.

Why not just use substring... string.substring(0, 7); The first argument (0) is the starting point. The second argument (7) is the ending point (exclusive). More info here.

var string = "this is a string";
var length = 7;
var trimmedString = string.substring(0, length);

Copying Will's comment into an answer, because I found it useful:

var string = "this is a string";
var length = 20;
var trimmedString = string.length > length ? 
                    string.substring(0, length - 3) + "..." : 
                    string;

Thanks Will.

And a jsfiddle for anyone who cares https://jsfiddle.net/t354gw7e/ :)

I suggest to use an extension for code neatness. Note that extending an internal object prototype could potentially mess with libraries that depend on them.

String.prototype.trimEllip = function (length) {
  return this.length > length ? this.substring(0, length) + "..." : this;
}

And use it like:

var stringObject= 'this is a verrrryyyyyyyyyyyyyyyyyyyyyyyyyyyyylllooooooooooooonggggggggggggsssssssssssssttttttttttrrrrrrrrriiiiiiiiiiinnnnnnnnnnnnggggggggg';
stringObject.trimEllip(25)

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/substr

From link:

string.substr(start[, length])