Add property to object when it’s not null

You could use Object.assign in combination with the ternary operator:

let data = Object.assign({},
  first === null ? null : {first},
  ...
);

This works because Object.assign will skip over null parameters.

If you are sure that the property value is not going to be “falsy”, then it would be bit shorter to write:

let data = Object.assign({},
  first && {first},
  ...
);

Assuming the object is going to be stringified at some point, since stringification ignores undefined values, you could also try

let data = {
  first: first === null ? undefined : first,
  ...
}