JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = [{
    id: 1,
    username: 'fred'
}, {
    id: 2,
    username: 'bill'
}, {
    id: 3,
    username: 'ted'
}];

function add(name) {
    var id = arr.length + 1;
    var found = arr.some(function (el) {
      return el.username === name;
    });
    if (!found) {
        arr.push({ id: id, username: name });
    }
}

add('ted');
console.log(arr);