IndexOfAll

Returns all indexes of a given element in the collection, or [-1] if it is not present.

• Use reflect.ValueOf() to get the array or slice and the value to search for, make() to create an appropriate slice.
• Use a for loop with Value.Len() and Value.Index() to iterate over each element and compare it to the search value.
• Use append() to add indexes of matching elements to the resulting slice.

import "reflect"

func IndexOfAll(params ...interface{}) []int {
    arr, v, r := reflect.ValueOf(params[0]),
        reflect.ValueOf(params[1]).Interface(),
        make([]int, 0)

    for i := 0; i < arr.Len(); i++ {
        if arr.Index(i).Interface() == v {
            r = append(r, i)
        }
    }
    if len(r) != 0 {
        return r
    }
    return []int{-1}
}

IndexOfAll([]int{1, 2, 1, 3, 1, 1}, 1) // [0 2 4 5]
IndexOfAll([]int{1, 2, 1, 3, 1, 1}, 4) // [-1]