Index can be an expression

let x = ["photon", "bosons", "proton", "muon", "tau"]
        //  0         1          2        3      4

console.log( x[0] )     //photon
console.log( x[1+2] )   // x[3] --> muon

let y = 4
console.log( x[y] )     // x[4] --> tau

Syntax

arrayVariable[expression]