var ratio = "2:1";  
 
main();   
function main(){   
if(!documents.length) return;   
var strtRulerUnits = app.preferences.rulerUnits;   
app.preferences.rulerUnits = Units.PIXELS;   
var W = activeDocument.width;;   
var H = activeDocument.height;   
 
ratio = ratio.split(":");   
if(W<H) ratio=ratio.reverse();   
var Hpics =(W/Number(ratio[0]))*Number(ratio[1]);   
if(Hpics > H){   
    Wpics = (H/Number(ratio[1]))*Number(ratio[0]);   
    Hpics = H;   
    }else{   
        Wpics = W;   
        }   
var Left,Top,Right,Bottom=0;   
if (W==Wpics) Left = 0; Right = W;    
if (W!=Wpics) {   
    var diffw = (W-Wpics)/2;   
    Left = diffw; Right = Wpics+diffw;   
    }   
if (H==Hpics) Top = 0; Bottom = H;   
if( H!=Hpics){   
    var diffh = (H-Hpics)/2;   
    Top = diffh; Bottom = Hpics+diffh;   
    }   
activeDocument.selection.select([[Left,Top],[Right,Top],[Right,Bottom],[Left,Bottom]], SelectionType.REPLACE, 0, false);    
executeAction(charIDToTypeID( "Crop" ), new ActionDescriptor(), DialogModes.NO );   
activeDocument.selection.deselect(); 
app.preferences.rulerUnits = strtRulerUnits;   
}