let arr1=[{"id":"1"},{"id":"2"},{"id":"3"}];
let arr2=[{"user":"admin"},{"user":"admin2"}];
const len = arr1.length > arr2.length ? arr1.length : arr2.length;
let arr3=[];
for(let i=0;i<len;i++){

arr3.push(Object.assign(arr1[i] || {},arr2[i] || {}))

}

var array1 = [{"id":"1"},{"id":"2"}];
var array2 = [{"user":"admin"},{"user":"admin2"}];
array1.map(function(elem, index) {
    elem = Object.assign(elem,array2[index]);
    return elem;
})
console.log(array1);