How convert string '+'(plus) in to operation +

let mass = ['+','-','/','*']; console.log(Number(mass[i]))// + - / *

const mass = ['+','-','/','*'];
let i = 0;
console.log(eval(`5${mass[i]}4`)) // 9
i = 1;
console.log(eval(`5${mass[i]}4`)) // 1
i = 2;
console.log(eval(`5${mass[i]}4`)) // 1.25
i = 3;
console.log(eval(`5${mass[i]}4`)) // 20