Depends on the form that `str = ch + str` takes inside the loop. But yeah, the way they wrote the code makes it less likely to work well.

    let padding = '';
    for (let i = 0; i < len; ++i)
      padding += ch;
    return padding + str;

The above would probably get caught by the JIT and would essentially be optimized to what `ch.repeat(len) + str;` would do.