Go template remove the last comma in range loop

Question

I have code like this:

package main

import (
    "text/template"
    "os"
)

func main() {
    type Map map[string]string
    m := Map {
        "a": "b",
        "c": "d",
    }
    const temp = `{{range $key, $value := $}}key:{{$key}} value:{{$value}},{{end}}`
    t := template.Must(template.New("example").Parse(temp))
    t.Execute(os.Stdout, m)
}

it will output :

key:a value:b,key:c value:d,

but I want something like this:

key:a value:b,key:c value:d

I don't need the last comma, how to remove it. I found a solution for looping an array here: https://groups.google.com/d/msg/golang-nuts/XBScetK-guk/Bh7ZFz6R3wQJ , but I can't get index for a map.

Answer 1

Since Go 1.11 it is now possible to change values of template variables. This gives us the possibility to do this without the need of a custom function (being outside of the template).

The following template does that:

{{$first := true}}
{{range $key, $value := $}}
    {{if $first}}
        {{$first = false}}
    {{else}}
        ,
    {{end}}
    key:{{$key}} value:{{$value}}
{{end}}

Here's the altered working example from the question:

type Map map[string]string
m := Map{
    "a": "b",
    "c": "d",
    "e": "f",
}
const temp = `{{$first := true}}{{range $key, $value := $}}{{if $first}}{{$first = false}}{{else}}, {{end}}key:{{$key}} value:{{$value}}{{end}}`
t := template.Must(template.New("example").Parse(temp))
t.Execute(os.Stdout, m)

Which outputs (try it on the Go Playground):

key:a value:b, key:c value:d, key:e value:f

Answer 2

I used the below template to correctly format some JSON using a go template.

It handles a collection of any size or empty.

{{- define "JoinList" -}}
   {{- $lastIndex := math.Sub (len .) 1 -}}
   {{- range $index, $property := . -}}
   {{ if isKind "string" $property.value }}
   "{{ $property.key }}": "{{ $property.value }}"{{ if ne $index $lastIndex }},{{ end }}
   {{- else -}}
   "{{ $property.key }}": {{ $property.value }}{{ if ne $index $lastIndex }},{{ end }}
   {{ end }}
   {{- end -}}
{{- end -}}