Javascript - Leading zero to a number converting the number to some different number. not getting why this happening?

Question

A leading zero to some number converting the number to some unknown number format. for example :

017 is getting converted to 15

Answer 1

One of the reasons to "use strict";

(function() {"use strict"; 017})()

// Firefox => SyntaxError: "0"-prefixed octal literals and octal escape sequences are deprecated; for octal literals use the \"0o\" prefix instead 
// Chrome, Node => SyntaxError: Octal literals are not allowed in strict mode.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Deprecated_octal