Try: pixel_size.selection.text === “1px”
Like this:
win=new Window("palette","The Fluent Shadow",undefined,{resizeable:true,independent:false,minimizeButton:false,maximizeButton:false,resizeable:false,});
but_1=win.add("button",[20,60,120,80],"Fluent Shadow");
but_1.helpTip="Select a layer";
pixel_size=win.add("dropdownlist",[20,20,120,42] ,["1px","2px","3px","4px","6px","8px","9px","12px","16px","24px","32px","48px","64px","80px","96px",""]);
pixel_size.selection = 0;
pixel_size.helpTip="Choose your Shadow size";
win.center();
win.show();
but_1.onClick = function(){
if(pixel_size.selection.text === "1px"){
alert("1px");
}
if(pixel_size.selection.text === "2px"){
alert("2px");
}
if(pixel_size.selection.text ==="3px"){
alert("3px");
}
}