How to script if / then

Thank you very much. That work as is. However when I add other code to the try section it give me "Error 16: catch/finally without try". Basically looking to see if there is a Dimension swatch. If so select everything with that stroke color and move it to the Dimensions layer.

var idoc = app.activeDocument;
var swatches = idoc.swatches;

try {
    var mySwatch = swatches.getByName("Dimension");
//    alert ("swatch found");

    var mydoc = app.activeDocument;
    mydoc.selection = null;

    mydoc.defaultStrokeColor = mydoc.swatches["Dimension"].color;
    app.executeMenuCommand("Find Stroke Color menu item");

    var doc = app.activeDocument;
    var layerName = 'Dimensions';
    var _layer = doc.layers.getByName(layerName);
    var _selectedItems = app.selection
    for (var i = _selectedItems.length - 1; i >= 0; i--) {
        _selectedItems[i].move(_layer, ElementPlacement.PLACEATEND);
        _selectedItems[i].selected = false;
    app.redraw();

}

catch (e) {
    alert ("swatch not found")
}