JavaScript: Why when storing an array of an array in a variable, and pushing to that array a value, the original array increases in value too? [duplicate]

I wish I could find an answer to this question or anything that points me to the right direction. Assume below scenario

let x = [[1,2,3],[2,3,4]]
let y = x[0]
y.push(66)
console.log(x[0]) 
// [1,2,3,66]

How can I store in the “y” variable the value of x[0], add an element to Y and maintain X to its original roots?

Also why exactly does x[0] gain the new value? I’m no where doing x[0].push(value)