Function reassigns argument that is passed to it unintentionally. (Node.js / Mongo )

I'm writing a program using Epxress, Node and Mongo that generates sudoku's and allows people to fill it in. In the code below, I first run the genSud() method, which generates a complete sudoku. Then, I run the emptySud() method, which takes the complete sudoku from genSud(), and a difficulty level. A difficulty level of 20 would mean the emptySud() method takes out 20 numbers from the full sudoku. After this, I try to add both the half-empty sudoku (for showing to the user), and the full sudoku (for checking the user's input) to the database. However, for some reason, when I pass the full Sudoku into the emptySud() method, the full Sudoku takes on the value of the ultimate result of emptySud(). Essentially, I end up with twice the half sudoku in the database, rather than the full one and the half one.

I suspect this has to do with linkage, which is why I use "use strict", but to no avail. Any ideas? i have created a CodePen that allows you to see how the code runs for itself. The genSud() returns a full sudoku, the emptySud() removes some numbers. As you can see, they run fine separately, but somehow I can't get them to work together whilst saving both a full and a half sudoku.

https://fiddle.jshell.net/pjxowsdh/

The code below only shows how I run the functions from the main 'app.js'.

 let completeSudoku = sudoku.genSud();
 // console.log(completeSudoku) returns a full sudoku
 let tempSudoku = completeSudoku;
 // console.log(completeSudoku) returns a full sudoku
 let halfSudoku = sudoku.emptySud(tempSudoku, difficulty);
 // console.log(completeSudoku) returns the half sudoku