Notes: Golang generics with map, filter, and reduce implementation

In this blog blog, we’ll see how Go generics can be used to write popular JS array methods.

Map (Transform each element in a slice)

func Map[T any, R any](slice []T, fn func(T) R) []R {
    result := make([]R, len(slice))
    for i, v := range slice {
        result[i] = fn(v)
    }
    return result
}

Usage:

numbers := []int{1, 2, 3, 4}
doubled := Map(numbers, func(n int) int { return n * 2 })
fmt.Println(doubled) // [2 4 6 8]

words := []string{"go", "generics"}
lengths := Map(words, func(s string) int { return len(s) })
fmt.Println(lengths) // [2 8]

Filter (Select elements matching a condition)

func Filter[T any](slice []T, fn func(T) bool) []T {
    var result []T
    for _, v := range slice {
        if fn(v) {
            result = append(result, v)
        }
    }
    return result
}

Usage:

numbers := []int{1, 2, 3, 4, 5, 6}
evens := Filter(numbers, func(n int) bool { return n%2 == 0 })
fmt.Println(evens) // [2 4 6]

words := []string{"go", "rust", "java", "python"}
shortWords := Filter(words, func(s string) bool { return len(s) <= 4 })
fmt.Println(shortWords) // ["go", "rust", "java"]

Reduce (Aggregate elements into a single value)

func Reduce[T any, R any](slice []T, initial R, fn func(R, T) R) R {
    result := initial
    for _, v := range slice {
        result = fn(result, v)
    }
    return result
}

Usage:

numbers := []int{1, 2, 3, 4}
sum := Reduce(numbers, 0, func(acc, n int) int { return acc + n })
fmt.Println(sum) // 10

words := []string{"a", "b", "c"}
concat := Reduce(words, "", func(acc, s string) string { return acc + s })
fmt.Println(concat) // "abc"

Combining Them

numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}

// filter even numbers → double them → sum them
result := Reduce(
    Map(
        Filter(numbers, func(n int) bool { return n%2 == 0 }),
        func(n int) int { return n * 2 },
    ),
    0,
    func(acc, n int) int { return acc + n },
)

fmt.Println(result) // 2+4+6+8 → 20 (after doubling: 4+8+12+16 → 40)