How to Perform Calculations in Go HTML Templates?

How to Perform Calculations Within HTML Templates?

Problem:

In Go HTML templates, attempting to perform calculations like {{ $length -1 }} within a template does not work. How can we achieve this functionality?

Answer:

Unfortunately, calculations cannot be performed directly within HTML templates. This is due to the fundamental design philosophy of separating complex logic from templates.

Solutions:

Instead, consider these alternative approaches:

1. Pass Calculated Results as Parameters:

The preferred method is to calculate the desired value in your Go code and pass it as a template parameter.

func main() {
    length := len(myMap)
    t := template.Must(template.New("index").Parse(`<p>The last index of this map is: {{ .LastIndex }}</p>`))
    t.Execute(os.Stdout, map[string]interface{}{"LastIndex": length - 1})
}

2. Register Custom Functions:

You can also register custom functions and call them within your templates. These functions can perform calculations and return values.

func RegisterCalcFunc(t *template.Template) {
    t.Funcs["calcIndex"] = func(length int) int { return length - 1 }
}

func main() {
    RegisterCalcFunc(t)
    t := template.Must(template.New("index").Parse(`<p>The last index of this map is: {{ calcIndex .Length }}</p>`))
    t.Execute(os.Stdout, map[string]interface{}{"Length": len(myMap)})
}

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