Scope of a variable in Go (Golang) - Welcome To Golang By Example

Scope of a Variable (Local and Global Variable)

A variable declaration can be done at the package level or a function level or a block level. The scope of a variable defines from where that variable can be accessed and also the lifetime of the variable. Golang variables can be divided into two categories based on the scope.

Local Variable

Global Variable

Local Variable:

Local variables are variables which are defined within a block or a function level

An example of a block is a for loop or a range loop etc.

These variables are only be accessed from within their block or function

These variables only live till the end of the block or a function in which they are declared. After that, they are Garbage Collected.

A local once declared cannot be redeclared within the same block or function.

See below example

Local or loop variable i is not available after the for loop

Similarly local variable aaa is not available outside the function in which it is declared.

Hence below program will raise a compiler error

undefined: i
undefined: aaa.  #this occurs in the testLocal() function

Code:

package main

import "fmt"

func main() {
    var aaa = "test"
    fmt.Println(aaa)
    for i := 0; i < 3; i++ {
        fmt.Println(i)
    }
    fmt.Println(i)
}

func testLocal() {
    fmt.Println(aaa)
}

Global Variable

A variable will be global within a package if it is declared at the top of a file outside the scope of any function or block.

If this variable name starts with a lowercase letter then it can be accessed from within the package which contains this variable definition.

If the variable name starts with an uppercase letter then it can be accessed from an outside different package other than which it is declared.

Global variable are available throughout the lifetime of a program

For example, in the below program variable aaa will be a global variable available throughout the main package. It will be available in any function inside the main package. Do note that the variable name will not be available outside main package as its name starts with a lowercase letter.

package main

import "fmt"

var aaa = "test"

func main() {
    testGlobal()
}

func testGlobal() {
    fmt.Println(aaa)
}

Output:

test

Important Points

A variable declared within an inner scope having the same name as variable declared in the outer scope will shadow the variable in the outer scope.

package main

import "fmt"

var a = 123

func main() {
    var a = 456
    fmt.Println(a)
}

Output:

golang

scope

variable