mutable-unshift-method.md

const droids = ['R2-D2', 'C-3PO', 'K-2SO']

// define newDroids with droids reference
const newDroids = droids

// add 'BB-8' value to start of the new array created
newDroids.unshift('BB-8')

// the original variable is updated too, because it is the original reference
console.log(droids)
// ['BB-8', 'R2-D2', 'C-3PO', 'K-2SO']

// variable newDroids was updated by the reference
console.log(newDroids)
// ['BB-8', 'R2-D2', 'C-3PO', 'K-2SO']