why output of 3.5 +020 is 19.5 in JavaScript?

Answer 1

Hello Al Sirang
The octal numbers are represented by adding a 0 as prefix to them..
Just like we use 0x as prefix for hexadecimal.

020 is octal equivalent of 16

You can find this value by repeated division by base 8 method or
Try toString method and get octal value of decimal 16

alert((16).toString(8))

Result of this will be 20

I hope you are clear why 3.5+16 =19.5

:) happy learning

refer :
https://en.m.wikipedia.org/wiki/Octal#In_computers

Answer 2

decimal literals can start with a zero (0) followed by another decimal digit, but if every digit after the leading 0 is smaller than 8, the number gets parsed as an octal number. so:

"020" parsed as octal in non-strict mode (16 in decimal)