Javascript for pushing only unique values in array

JavaScript to give error on pushing a duplicate value in an array

  var fileArray= [];

var file1=attachment[0].files[0].val();

//somewhere else

function insertUniqueAttachment(file1){
  if(!fileArray.includes(file1)){
    fileArray.push(val)
  }else{
    alert("can't contain duplicate value");
  }
}