Length

let x = ["dog", "cat", "hamster", "fish"]
        // 0     1       2            3   

console.log( x.length )     //4

Expression

let x = ["dog", "cat", "hamster", "fish"]
let y = x.length
let z = y-1

console.log( x[z] ) // ?

Simplified

let x = ["dog", "cat", "hamster", "fish"]

console.log( x[x.length-1] ) // ?