The simplest way would be to use the native Number function:

var x = Number("1000")

If that doesn't work for you, then there are the parseInt, unary plus, parseFloat with floor, and Math.round methods.

parseInt()

var x = parseInt("1000", 10); // You want to use radix 10
    // So you get a decimal number even with a leading 0 and an old browser ([IE8, Firefox 20, Chrome 22 and older][1])

Unary plus

If your string is already in the form of an integer:

var x = +"1000";

floor()

If your string is or might be a float and you want an integer:

var x = Math.floor("1000.01"); // floor() automatically converts string to number

Or, if you're going to be using Math.floor several times:

var floor = Math.floor;
var x = floor("1000.01");

parseFloat()

If you're the type who forgets to put the radix in when you call parseInt, you can use parseFloat and round it however you like. Here I use floor.

var floor = Math.floor;
var x = floor(parseFloat("1000.01"));

round()

Interestingly, Math.round (like Math.floor) will do a string to number conversion, so if you want the number rounded (or if you have an integer in the string), this is a great way, maybe my favorite:

var round = Math.round;
var x = round("1000"); // Equivalent to round("1000", 0)