generateUniqueIdentifier，Function declaration is problematic

[tangdiying]

generateUniqueIdentifier?: () => string;

generateUniqueIdentifier:function(file, event){
var custom = $.getOpt('generateUniqueIdentifier');
if(typeof custom === 'function') {
return custom(file, event);
}
var relativePath = file.webkitRelativePath||file.fileName||file.name; // Some confusion in different versions of Firefox
var size = file.size;
return(size + '-' + relativePath.replace(/[^0-9a-zA-Z_-]/img, ''));
},

Function declaration is problematic