As the name suggests, init() is used to initialize your program with requirements for its lifecycle. It is a powerful tool for several common use cases.

Common Use Cases:

1. Database Connections: Establishing an initial connection pool.
2. Cloud Credentials: Exporting or validating cloud-specific environment variables.
3. Static Configurations: Fetching or parsing configuration files.
4. Logging: Setting up global loggers and log levels for a package.
5. Global State: Defining global variables within a package.

Key Attributes of the init() function:

• Scoped to Package: Its scope is limited to the package block where it is defined.
• Called Once: It is executed exactly once per package.
• Implicit Execution: Go handles the execution of init() automatically; you don’t call it manually.
• No Arguments/Returns: It takes no parameters and returns no values.
• Order of Execution: Go supports multiple init() functions within the same package, but their execution order is calculated alphabetically by filename and then by order of declaration.

Anything initialized within an init() function is exported within the scope of that package. For a deep dive, refer to the Official Go Package Initialization Docs.

Multiple init() functions

It is possible to define multiple init() functions in a single file or across multiple files within the same package.

Example:

package main

import (
    "fmt"
)

var Var1 string
var Var2 string

func init() {
    fmt.Println("First Init Function Called")
    Var1 = initialize()
}

func init() {
    fmt.Println("Second Init Function Called")
    Var2 = Var1
}

func main() {
    fmt.Printf("Going by the order and printing value of Var2: [%s]\n", Var2)
}

func initialize() string {
    return "Executed by Init Function"
}

Result:

> go run initFunctions.go
First Init Function Called
Second Init Function Called
Going by the order and printing value of Var2: [Executed by Init Function]

Using init() for Side Effects

Sometimes we need a package to be in a certain state even if we don’t use its exported functions. This is known as importing for side effects.

import _ "github.com/go-sql-driver/mysql"

Go normally forbids unused imports. However, by using the blank identifier (_), we can force the package’s init() function to execute (e.g., to register a database driver) without referencing the package elsewhere in our code.

While init() functions are handy, they should not be overused as they can make the control flow harder to follow.

Happy Coding!!