Format a number as 2.5K if a thousand or more, otherwise 900

I need to show a currency value in the format of 1K of equal to one thousand, or 1.1K, 1.2K, 1.9K etc, if its not an even thousands, otherwise if under a thousand, display normal 500, 100, 250 etc, using JavaScript to format the number?

from Format a number as 2.5K if a thousand or more, otherwise 900