What am I missing with the understanding of Arrays?

Question

When arrays are assigned to another variable, the reference is passed as opposed to values. This is confirmed when you compare two arrays using == operator and it returns true

var a = [[1,2],[3,4],[5,6]];
var b = a; // b = [[1,2],[3,4],[5,6]]
var c = [].concat(a); // c = [[1,2],[3,4],[5,6]]

a == b; //true
a == c; //false

With the above inputs, when I modify the array b, it mutates array a, but not c.

b.push([7,8]); // b = [[1,2],[3,4],[5,6], [7,8]]
a; //a = [[1,2],[3,4],[5,6], [7,8]]
c; //c = [[1,2],[3,4],[5,6]]

But when I do the below, it mutates c.

b[0].push(5); // b = [[1,2,5],[3,4],[5,6], [7,8]]
a; //a = [[1,2,5],[3,4],[5,6], [7,8]]
c; //c = [[1,2,5],[3,4],[5,6]]

Why does this happen? This behavior happens with the usage of array methods that mutates the array.

Answer 1

.concat() does a shallow copy. So after the line:

var c = [].concat(a);

c and a reference different arrays, but c[0] and b[0] and a[0] all reference the same array.

Quoting from MDN:

concat copies object references into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays.