https://forums.adobe.com/thread/2583347

showAllLayers

    var aDoc = app.activeDocument;  
    var showLayers = function ( layers )  
    {  
        var i = layers.length, layer;  
        while ( i-- )  
        {  
        layer = layers[i];  
        layer.visible = true;  
        layer.layers.length && showLayers ( layer.layers );  
        }  
    }  
    showLayers ( aDoc.layers );