I have Javascript code that turns a whole string bold, however, I'd like to adapt the code to specifically bold only the part of the string that comes before the ':'

The code I currently have is:

```
function changeBrandName() 
{
  var prodList = document.querySelectorAll("h1.ProductList-title");
  for (i = 0, len = prodList.length; i < len; i++) 
  {
    var store = prodList[i].indexOf(":");

    for (j = 0; j < store; j++)
    {
      prodList[i].charAt[j].style.fontWeight = 'bold';
    } 
  }
}

```

Applying the fontWeight in this way isn't currently working - advice would be appreciated. 

The full string is "Vifa: Pebble Grey Oslo Loudspeaker" - I'd like 'Vifa' to be in bold.