Closures are functions that reference variables from outside their body. These variables are captured and remembered even after the outer function has finished executing.

closure_function.go

package main

import (
	"fmt"
)

// testFunction returns a closure. A closure is a function that captures variables from its surrounding scope.
// In this case, 'y' is captured and maintained between function calls.
func testFunction() func(int) int {
	var y int // 'y' is initialized in the enclosing function's scope.
	return func(x int) int { // Return an anonymous function that takes an int and returns an int.
		y += x // The anonymous function captures and modifies 'y'.
		return y // Return the updated value of 'y'.
	}
}

func main() {
	fFunc := testFunction() // fFunc is a closure returned by testFunction, with its own 'y' variable.
	for i := 0; i < 5; i++ {
		fmt.Println(fFive(1)) // Each call to fFunc(1) increments 'y' by 1 and prints the result.
	}
}

// Output:
// 1
// 2
// 3
// 4
// 5