const obj = {
    name: "some name",
    height: "182",
    mass: "77",
    hair_color: "black",
    skin_color: "red",
    eye_color: "brown",
    birth_year: "XXXXXX",
    gender: "female"
};

//just splice
jediArr.splice(3, 0, obj);
console.log(jediArr); // Array(11) [ {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, {…}, … ]

//splice via function
const pastItem = (arr, item) => {
  return arr.splice(3, 0, item);
};

console.log(pastItem(jediArr, obj)); // Array []