mongodb model contain changed on calling a function levelQuestion - mongodb

Question

Here is the code for schema
schema stores questions for a particular course and which contains chapter and there question
questions: [
{
Chapter: String,
chques: [
{
description: String,
questions: [
{
question: String,
options: [String],
answer: Number,
Explanation: String,
code: { type: String, default: null },
images: [{ type: String, default: null }],
level: String
}
]
}
]
}
],
Here is the code for route
Router.get(
"/queformock/:course_id",
passport.authenticate("jwt", { session: false }),
(req, res) => {
Courses.findOne({ _id: req.params.course_id })
.lean()
.exec()
.then(course => {
if (!course) res.status(404).json({ course: "course not found" });
else {
let coursepass = Object.assign({}, course);
console.log("coursepass1: ", coursepass); //before calling levelQuestion it output as expected
let level2 = levelQuestion(coursepass, "medium");
console.log("coursepass2: ", coursepass);
let level3 = levelQuestion(coursepass, "hard");
console.log("coursepass3: ", coursepass);
res.json(level2);
}
});
}
);
Here is the levelQuestion function code
function levelQuestion(coursed, type) {
let arr = [];
coursed.questions.forEach(item => {
item.chques.forEach(i => arr.unshift(i));
});
arr.forEach(item => {
_.remove(item.questions, i => {
return i.level !== type;
});
});
_.remove(arr, item => {
return item.questions == false;
});
return arr;
}
now the problem is on every function call of levelQuestion coursepass is changed...why??