Global Variable Scope in Go Templates

When creating global variables in Go's html/template package, understanding scope limitations is crucial. As per the package documentation, variables defined in a template have a limited scope extending to the end of the control structure (e.g., if, with, range) in which they are declared.

Case Study

Consider the following HTML/template code:

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

This code aims to display the current user ID inside the if block and 0 outside the block. However, the result shows 0 in both places due to the limited scope of $currentUserId.

Solution using Assignment

Go 1.11 introduced support for modifying template variable values. To initialize a variable, use :=, as in:

{{$currentUserId := 0}}

To update its value, use =, as in:

{{$currentUserId = .UserData.UserId}}

By modifying a variable defined outside the if block, the change can be accessed both inside and outside the block.

Alternative Workarounds

If modifying global variables is not suitable, consider these alternatives:

• Custom Function: Register a custom function that accepts a variable and assigns a value to it. This function can be called within the template to achieve a similar effect.
• Simulated Changeable Variables: Define a variable in the template data map and register a custom function to modify its value. This allows for simulated "changeable" variables within the template.

By leveraging these solutions, you can create and modify global variables in Go's html/template package, ensuring appropriate variable scoping and achieving desired functionality in your templates.