How to recursively copy arrays

I want to recursively produce the vertices (points) of a unit n-hypercube (for the sake of clarity, just a cube here). The idea is to specify the points of the cube by recursively going through the x, y, and z components. Why not just use nested for-loops? Recursion is intrinsically swag. This is my function:

var points = [];

function makeCube(d, point) {
  console.log(d,point);
  
  if(d == 0) {
    points.push(point);
  } else {
    extension1 = [...point];
    extension1.push(0);

    extension2 = [...point];
    extension2.push(1);
    
    makeCube(d - 1, extension1);
    makeCube(d - 1, extension2);
  }
}

I call makeCube(3, []). The console reads this:

[Log] 3 – [] (0) (sketch.js, line 57)
[Log] 2 – [0] (1) (sketch.js, line 57)
[Log] 1 – [0, 0] (2) (sketch.js, line 57)
[Log] 0 – [0, 0, 0] (3) (sketch.js, line 57)
[Log] 0 – [0, 0, 1] (3) (sketch.js, line 57)
[Log] 1 – [0, 0, 1] (3) (sketch.js, line 57)
[Log] 0 – [0, 0, 1, …] (4) (sketch.js, line 57)
etc...

When d is 1, there should only be 2 entries in the array, not 3! When d is 0, there should be 3, not 4. The second array in the lowest level is being carried up a level somehow. I made a point of copying the extension arrays rather than setting them equal to the old one, so I don’t understand what’s going on.