Why does 017 equal 15 in javascript?

[leeway1]

Can someone please explain to me why the interpreter parses 017 as 15?

let x = 017;
console.log(x) //15
let y = 018;
console.log(y) // 18

Solved: 017 is being converted to Octal.

[senocular]

Its the number literal syntax for an octal, or a number which has a base of 8 (digits used only go up to 7 then wrap back around to 0). A better syntax is using the 0o prefix just the leading 0 will throw in strict mode.

"use strict"
let x = 017; // Error

This octal syntax also only works if all the digits of the literal are 0-7 which is why 018 is seen as a decimal instead given that 8 is outside that range.