How to Format Floating-Point Numbers in Go HTML Templates?

Formatting Floats in Go HTML Templates

In Go HTML templates, you may need to format floating-point values for display. As you mentioned, converting a float64 to a string with strconv.FormatFloat in your .go file is straightforward. However, this approach is not suitable for templates.

Instead, you have several options to format floats within your HTML templates:

• Option 1: Before populating the template, format the number using fmt.Sprintf in your .go file.
• Option 2: Create a custom data type and implement the String() method to format the float according to your requirements.
• Option 3: Directly call fmt.Sprintf within the template, specifying the custom format string.
• Option 4: Register a custom function within the template to format the float using your preferred format string.

Here's an example code snippet that demonstrates these options:

import (
    "fmt"
    "html/template"
    "os"
)

// MyFloat is a custom type for formatting floats.
type MyFloat float64

// String implements the String() method for converting MyFloat to a string.
func (mf MyFloat) String() string {
    return fmt.Sprintf("%.2f", float64(mf))
}

func main() {
    t, err := template.New("").Funcs(template.FuncMap{
        // Register the MyFormat function to format floats using a custom format string.
        "MyFormat": func(f float64) string { return fmt.Sprintf("%.2f", f) },
    }).Parse(templ)
    if err != nil {
        panic(err)
    }

    m := map[string]interface{}{
        "n0": 3.1415,
        "n1": fmt.Sprintf("%.2f", 3.1415),
        "n2": MyFloat(3.1415),
        "n3": 3.1415,
        "n4": 3.1415,
    }

    if err := t.Execute(os.Stdout, m); err != nil {
        panic(err)
    }
}

const templ = `
Number:         n0 = {{.n0}}
Formatted:      n1 = {{.n1}}
Custom type:    n2 = {{.n2}}
Calling printf: n3 = {{printf "%.2f" .n3}}
MyFormat:       n4 = {{MyFormat .n4}}`

When you run this code (e.g., on the Go Playground), you will see the following output:

Number:         n0 = 3.1415
Formatted:      n1 = 3.14
Custom type:    n2 = 3.14
Calling printf: n3 = 3.14
MyFormat:       n4 = 3.14

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