...if variable exist...

If you mean that literally, the variable may not be declared, then you need to move the typeof check to the front in order to avoid trying to read the value of an undeclared variable. You can also take advantage of the fact that spreading undefined, null, or a boolean (like false) doesn't do anything (including not causing an error).

Here I've added a data3 that doesn't exist to show that part in action:

let data1 = {
    data1: "data1",
};

let data2 = {
    data2: "data2",
};

const items = {
    ...typeof data1 === "object" && data1,
    ...typeof data2 === "object" && data2,
    ...typeof data3 === "object" && data3,
};

console.log(items);

How that works (using data1 as the example):

1. typeof data1 === "object" tells us two things:
   • That there's an in-scope identifier called data1 (which means we can read its value without worrying about getting a ReferenceError for an undeclared variable)
   • That data1's value is of type "object"
2. That combined with && data1 means we'll be using one of these values with spread:
   • false if typeof data1 === "object" is false, or
   • The value of data1, which is either an object or null

Spreading null doesn't do anything, and spreading false doesn't do anything, so we either get no properties from that spread operation or we get the own, enumerable properties of an object that data1 references.