JSFiddle - React, Tailwind, and code Playground

JavaScript

var user_arr = new Array();

for (var i=0; i<20; i++) {
    user_arr[i] = get_unique_value(user_arr);
}


function get_unique_value(array) {
    var userran = parseInt((Math.random() * (123 - 0 + 1)), 10) + 0;
    while (alreadyExists(array, userran)) {
        userran = parseInt((Math.random() * (123 - 0 + 1)), 10) + 0;
    }
    return userran;
}

function alreadyExists(array, number) {
    for (var i=0; i<array.length; i++) {
        if (array[i]==number) {
            return true;
        }
    }
}


alert(user_arr);