JSFiddle - React, Tailwind, and code Playground

HTML

<pre><code id="output"></code></pre>

JavaScript

function removeDuplicates(arr) {
    var counts = arr.reduce(function(counts, item) {
        counts[item] = (counts[item]||0)+1;
        return counts;
    }, {});
    return Object.keys(counts).reduce(function(arr, item) {
        if(counts[item] === 1) {
            arr.push(item);
        }
        return arr;
    }, []);
}

var myArr = [1, 1, 2, 5, 5, 7, 8, 9, 9];
var newArr = removeDuplicates(myArr);
document.getElementById("output").textContent = JSON.stringify(newArr);