Number formatted with separator in Javascript [duplicate]

i have few numbers.

i want to format those number with comma(,)separator if number>999

But if number less than 999 it returns only number
99-->99

i want to get like below right side number.

9999.19-->9,999.19
899920.14--->8,99,920.14
2166593.35--->2,16,513.35

how can get these comma separator value if value greater than 999?