How Can I Manage and Modify Global Variables in Go HTML Templates?

Managing Global Variables in Go HTML/Template

When creating variables in Go's html/template, their scope is typically limited to the conditional block in which they're defined. To overcome this and share variables across multiple sections of a template, an alternative approach is required.

In Go 1.11, a new mechanism was introduced for modifying template variables.

Creating Global Variables:

To define a global variable, utilize the assignment operator (:=):

{{$globalVar := value}}

Modifying Global Variables:

To alter the value of a global variable, employ the assignment operator (=):

{{$globalVar = newValue}}

Use Case within Conditional Blocks:

If a global variable is created outside of an {{if}} block but modified within it, the changes become visible after the block.

{{$globalVar := 0}}
Before: {{$globalVar}}
{{if .UserData}}
    {{$globalVar = .UserData.UserId}}
    [<a href="#ask_question">Inside {{$globalVar}}</a>]
{{else}}
    {{$globalVar = 0}}
{{end}}
[<a href="#ask_question">Outside {{$globalVar}}</a>]

Example:

Consider the following template:

m := map[string]interface{}{}
t := template.Must(template.New("").Parse(src))

m["UserData"] = UserData{99}
if err := t.Execute(os.Stdout, m); err != nil {
    panic(err)
}

With the following source:

Before: {{$globalVar}}

    [<a href="#ask_question">Inside {{$globalVar}}</a>]

[<a href="#ask_question">Outside {{$globalVar}}</a>]

The output will be:

Before: 0

    [<a href="#ask_question">Inside 99</a>]

[<a href="#ask_question">Outside 99</a>]

This demonstrates the ability to modify global variables and have the changes reflected throughout the template.

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