# 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)**

```go
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:**

```go
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)**

```go
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:**

```go
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)**

```go
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:**

```go
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**

```go
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)
```

You can use these in your code as these make array operations super easy!