Number literal with leading zero does not result in expected value

Question

I have this code:

console.log(066); // 54

Why does it log 54, not 66?

Answer 1

In JavaScript, numeric literals that begin with a 0 are treated as octal.

From the MDN docs:

Octal number syntax uses a leading zero. If the digits after the 0 are outside the range 0 through 7, the number will be interpreted as a decimal number.

Answer 2

Because add a prefix 0 will make the number to be considered of base 8(octal), as way 0x will make the following number to be of base 16(hexa)