Do Go Template Range Loop Variables Reset on Each Iteration?

Variables in Go Template Range Loops: Are They Reset on Iteration?

In Go templates, variables declared outside a range loop are not reset on each iteration. However, an issue arises when assigning a new value to a variable within the loop.

Consider the following code:

{{ $prevDate := "" }}
{{ range $post := .Posts }}
    {{ if ne $prevDate $post.Date }}
        <div>

The intent is to compare the $prevDate to the current post's date to determine if the post occurred on the same day. However, $prevDate seems to be reset to an empty string at the start of each loop iteration.

The reason for this behavior is that the variable $prevDate is redeclared within the loop. This creates a new variable that is only in scope within the current iteration. The original $prevDate variable remains unchanged outside the loop.

To resolve this issue, there are two possible solutions:

Solution #1: Using a Registered Function

You can register a custom function that takes the current index and returns the previous post's date:

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

Then, in your template:

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

Solution #2: Using a Method of Posts

Alternatively, you can add a method to your Posts type:

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

In your template:

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

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