How to add objects within another array of object at alternate position?

For getting your desired array, you could make of use the concat method in combination with reduce.

var array = [{test: 1},{test: 2},{test: 3},{test: 4}];
var result = array.reduce((res, item) => res.concat(item, {dummy: 1}), []).slice(0, -1);
    
console.log(result);

Since this adds {dummy: 1} object after each element from your array, just use

.slice(0, -1)

to add one exception for the last element from your given array.

If you want to modify the original array inplace, you could use splice method.

var array = [{test: 1},{test: 2},{test: 3},{test: 4}];
for(i = 0;i < array.length - 1; i = i + 2){
  array.splice(i + 1, 0, {dummy: 1});
}
console.log(array);

To modify the original array (instead of creating an new one), you can first generate the indexes at which the new items need to be inserted, and then use Array.prototype.splice() :

const a = [{test: 1}, {test: 2}, {test: 3}, {test: 4}];

Array.from({length: a.length - 1}, (_, i) => i * 2 + 1)
     .forEach(i => a.splice(i, 0, {dummy: 1}));

console.log(a);

I like to use map if I can. It is easier to understand and maintain.

So first create an array of array and then flatten it a single array with flat.

let arr1 = [{test: 1},{test: 2},{test: 3},{test: 4}]

arr1 = arr1.map(item=>[item,{dummy:1}]).flat().slice(0, -1)

console.log(arr1);