Revealjs markdown, reveal code line by line

Question

I am making a Revealjs presentation based on markdown, and I want to reveal code line by line. I did the following :

    ```js [|1|1-2|1-3|1-4]
    let a = 1;
    let b = 2;
    let c = x => 1 + 2 + x;
    c(3);
    ```

But the issue is in the first highlight, it highlights everything and I want it to hide eveything instead. Is this a bug ? Or is there another way to accomplish what I want (First grey everything out, then show first line, and then show second line also, etc)

Thank you.