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How to Access Struct Fields in HTML Templates when Using Maps in Go?

Susan SarandonOriginal: 2024-10-24 07:43:29540browse

Accessing Struct Fields in HTML Templates with Go's html/template

In Go's html/template package, you can encounter challenges when accessing the fields of a struct that is stored as a value in a map. This article provides a solution to this problem, enabling you to retrieve and display individual fields of the struct within your templates.

Consider the following example, where we define a Task struct:

<code class="go">type Task struct {
   Cmd string
   Args []string
   Desc string
}</code>

We initialize a map with the Task struct as values and strings as keys:

<code class="go">var taskMap = map[string]Task{
    "find": Task{
        Cmd: "find",
        Args: []string{"/tmp/"},
        Desc: "find files in /tmp dir",
    },
    "grep": Task{
        Cmd: "grep",
        Args: []string{"foo", "/tmp/*", "-R"},
        Desc: "grep files match having foo",
    },
}</code>

Now, we want to parse an HTML page using the taskMap data using html/template:

<code class="go">func listHandle(w http.ResponseWriter, r *http.Request) {
    t, _ := template.ParseFiles("index.tmpl")
    t.Execute(w, taskMap)
}</code>

Here's the corresponding template, index.tmpl:

<code class="html"><html>
{{range $k, $v := .}}
   <li>Task Name: {{$k}}</li>
   <li>Task Value: {{$v}}</li>
   <li>Task Description: {{$v.Desc}}</li>
{{end}}
</html></code>

While accessing the $k and $v variables from the map works as expected, accessing the Desc field using {{$v.Desc}} fails. To resolve this, we need to ensure that the fields we want to access in the template are exported. In Go, fields are exported when they start with an uppercase letter.

Solution:

Modify the Task struct to export the Desc field:

<code class="go">type Task struct {
   Cmd string
   Args []string
   Desc string
}</code>

Update the map with the exported Desc field:

<code class="go">var taskMap = map[string]Task{
    "find": Task{
        Cmd: "find",
        Args: []string{"/tmp/"},
        Desc: "find files in /tmp dir",
    },
    "grep": Task{
        Cmd: "grep",
        Args: []string{"foo", "/tmp/*", "-R"},
        Desc: "grep files match having foo",
    },
}</code>

In the template, update the syntax to reference the exported Desc field:

<code class="html">{{range $k, $v := .}}
   <li>Task Name: {{$k}}</li>
   <li>Task Value: {{$v}}</li>
   <li>Task Description: {{$v.Desc}}</li>
{{end}}</code>

By following these steps, you will be able to access the struct fields in your HTML templates, allowing you to easily display and utilize the data stored in your Go maps.

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