WhiteBG.sketchplugin

// Give all artboards a white background (option+shift+w)
// (opt shift w)

// A new white layer will be created if a _Background layer doesn't already exist.
// This correctly handles if the _Background layer is already there, adjusts it for size/color.

var artboards = [[doc currentPage] artboards];

// Deselect all
if (selection && selection.count() > 0) {
  for(var k=0; k < selection.count(); k++) {
    selection[k].setIsSelected(false);
  }
}

if ([artboards count] > 0) {
  for (var i=0; i < [artboards count]; i++) {
    var ag = artboards[i];
    var frame = [ag frame];
    // Find if the background layer already exists or not.
    var layers = [ag layers];
    var rect = null;
    for (var j=0; j < [layers count]; j++) {
      var l = layers[j];
      if ([l name]+"" == "_Background") {
        log("Found existing");
        rect = l;
      }
    }

    if (rect == null) {
      rect = ag.addLayerOfType("rectangle");
      rect.setName("_Background");
    }

    // Set size
    [rect frame].width = [frame width];
    [rect frame].height = [frame height];

    // Set fill
    if (rect.style().fills()[0] != null) {
      rect.style().fills().removeStylePartAtIndex(0);
    }
    rect.style().fills().addNewStylePart();
    var color = rect.style().fills()[0].color();
    color.setRed(1);
    color.setGreen(1);
    color.setBlue(1);

    // Send to back
    rect.setIsSelected(true);
    [[doc actionWithName:"MSMoveToBackAction"] performAction:nil];
    rect.setIsSelected(false);
    rect.setIsLocked(true);
  }
}