JSFiddle - React, Tailwind, and code Playground

JavaScript

function format( num ){
    var postfixes = ['', 'K', 'M', 'B', 'T'], thousands = 0;
    while( num > 999 ){
       num = num / 1000;
       thousands += 1;
    }
    return parseFloat(
	        num.toString()
		    .replace(/(\.\d{2}).*/,"$1")
	    )
    .toFixed(2)
    .substr(0,4)
    .replace(/\.$/,'') + postfixes[ thousands ];
}

var tests = [ 0, 1.3456, 12, 529.96, 1001, 12385.123, 899999, 1000000, 2565898, 123456789, 1234567890 ];
tests.forEach( function( n ){
   console.log( "%f formatted as %s", n, format(n) ); 
});