How to Properly Reset Variables in Go Template Range Loops?

Resetting of Variables in Go Template Range Loops

In Go template range loops, variables declared outside the loop are not reset on each iteration. However, redeclaring a variable within the loop creates a new variable scoped only within that loop, potentially causing unexpected behavior.

Consider the following example:

{{ $prevDate := "" }}
{{ range $post := .Posts }}
    {{ if ne $prevDate $post.Date }}
        <div class="post-date">Posts dated: {{ $post.Date }}</div>
    {{ end }}
    <div class="post-content">{{ $post.Content }}</div>
    {{ $prevDate := $post.Date }}
{{ end }}

Here, $prevDate appears to be reset to an empty string at the start of each loop iteration. However, this is not due to a reset mechanism. Instead, a new variable $prevDate is being redeclared within the loop, overriding the outer variable.

Solution

To avoid this issue, it is important to utilize the correct scoping for variables in range loops. There are two recommended solutions:

Solution 1: Using a Registered Function

You can register a function that accepts the loop index and returns the desired value from the previous loop iteration. For instance:

func PrevDate(i int) string {
    if i == 0 {
        return ""
    }
    return posts[i-1].Date
}

// Registering the function:
var yourTempl = template.Must(template.New("").
    Funcs(map[string]interface{}{"PrevDate": PrevDate}).
    Parse(yourStringTemplate))

In your template, you can then call the function like this:

{{range $index, $post := .Posts}}
    {{$prevDate := PrevDate $index}}
{{end}}

Solution 2: Using a Posts Method

Alternatively, you can add a method to your Posts type that returns the previous date. For example:

type Post struct {
    // Your Post type
    Date string
}

type Posts []Post

func (p *Posts) PrevDate(i int) string {
    if i == 0 {
        return ""
    }
    return (*p)[i-1].Date
}

In your template, you can access the method like this:

{{range $index, $post := .Posts}}
    {{$prevDate := $.Posts.PrevDate $index}}
{{end}}

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