How to Remove Trailing Commas in Go Template Range Loops Without Custom Functions?

Removing Commas in Range Loop Templates with Go

In Go template loops, a comma often appears after each iteration, separating the output. For instance:

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

To eliminate this comma, a solution is sought that doesn't use custom functions.

Answer:

As of Go 1.11, it's possible to directly modify template variables. This enables comma removal through the following template:

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

Within this template, a variable $first is initialized to true. During the loop, it checks if this is the first iteration, and if so, sets $first to false to prevent commas on subsequent iterations.

Example:

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)

Output:

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

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