Проверьте на этой странице для объяснения slice (), split () и substring (), а также других функций String Object.
var num = 3874923.12 + ''; //converts to a string
numArray = num.split('.'); //numArray[0] = 3874923 | numArray[1] = 12;
commaNumber = '';
i = numArray[0].length;
do
{
//we don't want to start slicing from a negative number. The following line sets sliceStart to 0 if i < 0. Otherwise, sliceStart = i
sliceStart = (i-3 >= 0) ? i-3 : 0;
//we're slicing from the right side of numArray[0] because i = the length of the numArray[0] string.
var setOf3 = numArray[0].slice(sliceStart, i);
commaNumber = setOf3 + ',' + commaNumber; //prepend the new setOf3 in front, along with that comma you want
i -= 3; //decrement i by 3 so that the next iteration of the loop slices the next set of 3 numbers
}
while(i >= 0)
//result at this point: 3,874,923,
//remove the trailing comma
commaNumber = commaNumber.substring(0,commaNumber.length-1);
//add the decimal to the end
commaNumber += '.' + numArray[1];
//voila!