test.js

function checkIfFieldExists(ctx, list, fieldName, cb) {
  ctx.load(list.get_fields().getByInternalNameOrTitle(fieldName), 'Id');
  
  ctx.executeQueryAsync(function() {
    cb(true);
  }, function() {
    cb(false);
  });
}

var c = SP.ClientContext.get_current();
var l = c.get_web().get_lists().getByTitle('Test');
checkIfFieldExists(c, l, 'FailField', function(exists) {
  console.log('Field exists:', exists);
});