JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [1, 1, 1];
var arr2 = [1, 2, 3];

function checkIfThereIsOnlyOneElement(arr) {
    return arr.filter(function (item, i, ar) {
            return ar.indexOf(item) === i;
        }).length === 1;
}

console.log(checkIfThereIsOnlyOneElement(arr));
console.log(checkIfThereIsOnlyOneElement(arr2));