How Can I Define and Modify Global Variables in Go Templates?

Using Go Templates: Creating and Modifying Global Variables

Go templates offer convenient functionality for creating reusable code fragments for generating dynamic HTML. One common challenge encountered when working with templates is the inability to define and modify global variables across different sections of the template.

The Problem: Global Variables with Limited Scope

Consider the following HTML template:

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

Within the if condition, the template defines a local variable $currentUserId with a value assigned based on the UserData attribute. However, this variable's scope is limited to the if block, while we desire access to it outside as well.

The Solution: Defining and Modifying Variables

To define a global variable in a Go template, use the assignment operator :=.

{{$currentUserId := 0}}

To change the value of a variable, use the assignment operator =.

{{$currentUserId = .UserData.UserId}}

By defining the variable outside the if block but modifying it within the block, changes to $currentUserId persist after the if block.

For instance, the following code adds some additional logic before and after the if block:

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

Now, for both cases of UserData being present and absent, the $currentUserId variable is correctly updated and accessible outside the if block.

Other Approaches

In addition to using global variables, you can also consider:

• Custom functions: Registering custom functions allows you to run specific logic and access data from within your templates.
• Simulating changeable variables: By utilizing custom functions, you can simulate the behavior of changeable variables using a mechanism such as a map.

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