Замена delete на splice()

console.log(nonUniqueElements([1, 2, 3, 4, 5]))

function nonUniqueElements(data) {
let xix =[];
    
for (let i =0; i <data.length; i++) {
   
    if(data.indexOf(data[i], data.indexOf(data[i])+1) ==-1) delete data[i];
    
}
    for (let element of data) {
        if (element ==null) continue;
        xix.push(element);
    }
    return xix;
        }
 console.log(nonUniqueElements([1, 2, 3, 1, 3])) // [1,3,1,3]
 console.log(nonUniqueElements([1, 2, 3, 4, 5]))//[]
    console.log(nonUniqueElements([5, 5, 5, 5, 5]))//[5,5,5,5,5]
   console.log(nonUniqueElements([10, 9, 10, 10, 9, 8]))//[10,9,10,10,9]

function nonUniqueElements(data) {
let xix = Array.from(data);
    
for (let i =0; i <data.length; i++) {
   
    if(xix.indexOf(data[i], xix.indexOf(data[i])+1) ==-1) {xix.splice( xix.indexOf(data[i]), 1)}
    
};   
    return xix;
        }
 console.log(nonUniqueElements([1, 2, 3, 1, 3])) // [1,3,1,3]
 console.log(nonUniqueElements([1, 2, 3, 4, 5, 6]))//[]
 console.log(nonUniqueElements([5, 5, 5, 5, 5]))//[5,5,5,5,5]
 console.log(nonUniqueElements([10, 9, 10, 10, 9, 8]))//[10,9,10,10,9]

const nonUniq = arr => (obj => arr.filter(v => obj[v] > 1))(arr.reduce((o, v) => (o[v] = ++o[v] || 1, o), {}))

const nonUniqueElements = arr => {
    const obj = arr.reduce((o, v) => {
        o[v] = ++o[v] || 1
        return o
    }, {})
    return arr.filter(v => obj[v] > 1)
}