typescript cast number type to never [duplicate]

r = Math.floor(Math.random() * arrOfCourse.length) + 1;if (arr.indexOf(r) === -1) arr.push(r);

I got this error on the r Argument of type 'number' is not assignable to parameter of type 'never'.

I'm suppose to cast r ? If I try to do:r:any = Math.floor(Math.random() * arrOfCourse.length) + 1; nothing change...