The cellClass property defined in columnDefinitions is not working

I'm using ag-grid with no frameworks. The cellClass property defined in columnDefinitions is not working. I defined a function as shown below, but it is never called. I tested it in ag-grid-react and it was working fine.

cellClass: function (params) {
        console.log(params)
        if (params.colDef.colId === "") {
          return "border-top";
        }
        if (params.value === undefined) {
          return "border-right";
        }
      }