JSFiddle - React, Tailwind, and code Playground

JavaScript

var thelist = [1, 2, 3];

function addlist(data) {

    alreadyExists = thelist.some(function (item) {
        return item === data
    });

    if (!alreadyExists) {
        thelist.push(data);
    }
}
addlist(1);
addlist(2);
addlist(5);

console.log(thelist);