The splice function returns a deleted element or an empty array The unshift function return value is array length

Maybe,you can change this code

if (index > -1) {
  console.log('index', index);
  advantages.splice(index, 1);
  console.log('advantages', advantages.length);
  advantages.unshift(type);
  console.log('advantages', advantages.length);
  this.setPurchaseAdvantages(advantages);
}

Both of these functions can change the original array, so you don't need to create a new array to operate