Understanding the init Function in Go: Purpose, Execution, and Best Practices

Go init Function

In Go, the init function is a special function that is automatically executed before the main function when a package is initialized. It is primarily used for setup tasks, such as initializing global variables, opening database connections, or registering dependencies.

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Key Characteristics of init Function:

1. No Arguments & No Return Value – The init function does not take parameters or return values.
2. Executed Automatically – It runs before main() and does not require explicit invocation.
3. Can Have Multiple init Functions – A package can have multiple init functions, even across different files.
4. Executed in Declaration Order – If multiple init functions exist in a package, they are executed in the order in which they appear.

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Example 1: Basic init Usage

package main

import "fmt"

func init() {
    fmt.Println("Initializing...")
}

func main() {
    fmt.Println("Main function running...")
}

Output:

Initializing...
Main function running...

✅ The init function runs before main().

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Example 2: Using init for Global Variable Initialization

package main

import "fmt"

var config string

func init() {
    config = "Application Configured"
    fmt.Println("Configuring application...")
}

func main() {
    fmt.Println(config) // Output: Application Configured
}

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Example 3: init in Multiple Files

📌 If a package has multiple files, all init functions run before main(), in the order they appear.

File 1 (a.go)

package main

import "fmt"

func init() {
    fmt.Println("Init from a.go")
}

File 2 (b.go)

package main

import "fmt"

func init() {
    fmt.Println("Init from b.go")
}

Output (execution order is preserved):

Init from a.go
Init from b.go
Main function running...

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Example 4: init in a Different Package

package utils

import "fmt"

func init() {
    fmt.Println("Initializing utils package...")
}

func SayHello() {
    fmt.Println("Hello from utils!")
}

Main File (main.go)

package main

import (
    "fmt"
    "your_project/utils"
)

func init() {
    fmt.Println("Initializing main package...")
}

func main() {
    fmt.Println("Main function running...")
    utils.SayHello()
}

Output (Package-Level Execution Order)

Initializing utils package...
Initializing main package...
Main function running...
Hello from utils!

✅ The init function in imported packages runs before the init function in main.

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When to Use init?

✅ Good Use Cases:

• Initializing global variables.
• Setting up logging configurations.
• Registering dependencies (e.g., database connections).
• Ensuring required setup before main() runs.

❌ Avoid Using init for:

• Complex logic (prefer explicit initialization in main).
• Business logic (should be in main or other functions).

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Summary

• init() runs automatically before main().
• It has no parameters and no return values.
• Each package can have multiple init functions.
• init in imported packages runs before init in main.