Octal numbers can be expressed via numeric literals with leading 0 like 042, but this expression often confuses programmers. That's why ECMAScript's strict mode throws SyntaxError for the expression.

Since ES2015, the other prefix 0o has been introduced as an alternative. This new one is always encouraged to use in today's code.

Invalid:

const a = 042;
const b = 7 + 042;

Valid:

const a = 0o42;
const b = 7 + 0o42;
const c = "042";