JSFiddle - React, Tailwind, and code Playground

JavaScript

function uniques(arr) {
    if (arr.length === 1) { return arr };
    var a = [];
    for (var i = 0, l = arr.length; i < l; i++) {
        if (a.indexOf(arr[i]) === -1) {
            a.push(arr[i]);
        }
    }
    return a;
}

var arr = [1, 2, 3, 2, 1, 10, 11, 3, 2, 1];
console.log(uniques(arr))

var arr = [1];
console.log(uniques(arr))