JSFiddle - React, Tailwind, and code Playground

JavaScript

var s = 'jgtgkuvjghkpcnvguvecugvjcvaqwowuvfgetarv',
    alphabet = 'abcdefghijklmnopqrstuvwxyz',
    alphabetFrequency = 'e t a o i n s h r d l c u m w f g y p b v k j x q z'.split(' '),
    scores = {}

for (var i = 0; i < alphabet.length; i++) {
    scores[i] = getScore(rot(s, alphabet.length - i))
}
var minScore = Infinity, minScoreN = -1
for (var x in scores) {
    if (scores[x] < minScore) minScore = scores[x], minScoreN = x
}
console.log((minScoreN + 1) + ' ' + rot(s, alphabet.length - minScoreN))

function getScore(str) {
    var score = 0
    for (var i = 0; i < str.length; i++) {
        score += alphabetFrequency.indexOf(str.charAt(i))
    }
    return score
}

function rot(str, amount) {
    var rotated = ''
    for (var i = 0; i < str.length; i++) {
        rotated += alphabet[(alphabet.indexOf(str.charAt(i)) + amount) % alphabet.length]
    }
    return rotated
}