Practicing Go with Struct, Json, API and Templates

Introduction #

Go has four types of data structure, arrays, slices, maps and struct. In this tutorial, we will use struct. To get a json object, we’ll use my favorite api, JokeAPI. I use this whenever I am learning new technologies such as React, Flutter, or backend development.

Then, we will extract the json object from the http response and decode it to the struct. Finally, we’ll use template libraries to save these jokes as both text and HTML files.

JSON returned by the API #

var url string = "https://v2.jokeapi.dev/joke/Programming"

Declaring struct base on JSON #

A struct is like an object. It has fields and data types. The fields name are declared with capital letters to export them. To associate each field with json field, literal string is used.

type Joke struct {
	Category string `json:"category"`
	Type     string `json:"type"`
	Joke     string `json:"joke"`
	SetUp    string `json:"setup"`
	Delivery string `json:"delivery"`
	ID       int    `json:"id"`
	Safe     bool   `json:"safe"`
}

Notice One of the fields, Joke string, was named the same as the type Joke struct. It is a bad practice & poor choice of word. But I decided to keep it there to remind myself this type of things can happen. Plus, I wanted to keep the same field name as the json object returned by API call.

Write a function that fetch jokes #

func getJokes() Joke {
	response, err := http.Get(url)
	if err != nil {
		log.Fatal(err)
	}
	defer response.Body.Close()

	var joke Joke	
	json.NewDecoder(response.Body).Decode(&joke)

	return joke 
	// output: {Programming twopart  Why do programmers prefer using the dark mode? Because light attracts bugs. 232 true}
}

To get the json data, we declare a Joke variable. Then, create a NewDecoder object using response.Body. The NewDecoder object has a Decode() function that takes address to the Joke variable to compare JSON keys and extract values.

The output is a Go struct with curly brackets. It does not show JSON field name. We’ll use template packages later to format the output.

Calling the getJokes() in main() #

To fetch 10 jokes, let’s call the function in a forLoop. And append each joke into jokes slice.

	var jokes []Joke
	for i := 0; i < 10; i++ {
		joke := getJokes()
		jokes = append(jokes, joke)
	}

Declare template variables #

The joke json objects decoded in getJokes() are not neatly formatted. It is also unclear what field each json object contains. Depending on the type of joke, it can contain "joke" or "setup" & "delivery" fields. To format them, we’ll use templates.

Go’s standard library comes with a package for that. There are two types of templates, text/template & html/template. The later can be used for simple frontend application.

Inside the template, you will see double brackets, {{ }}, which are called action. They are very useful for accessing the object being passed to the template as dynamic data. That object can be access as "{{.}}", a period. In our case, the dot will represent Joke struct. Then, we can access the fields of the object with dot notation like this {{.Category}}.

IMPORTANT make sure the spelling is correct.

Text template #

The {{range .}} in the first line and {{end}} in the last line of code is simply a forLoop to iterate through all the fields of Joke struct. The dot represents a Joke struct.

var templ = `{{range .}}-------------------------
Category: {{.Category}}
Type: {{.Type}}
Joke: {{.Joke}} 
SetUp: {{.SetUp}}
Delivery: {{.Delivery}}
ID: {{.ID}}
Safe: {{.Safe}}
{{end}}`

HTML template #

var templ = `{{range .}}
<h3>Category: {{.Category}}</h3>
<h5>Type: {{.Type}}</h5>
<p>Joke: {{.Joke}}</p> 
<p>Setup: {{.SetUp}}</p>
<p>Delivery: {{.Delivery}}</p> 
<p>ID: {{.ID}},  Safe: {{.Safe}}</p>
{{end}}`

Parsing and Executing the templates #

	jk, err := template.New("joke").Parse(templ)
	if err != nil {
		log.Fatal(err)
	}
	report.Execute(os.Stdout, jokes)

Control flows inside templates #

You can also execute logical flow inside action such as forLoop and if/else statements. The previous text template is good, but can be improved. Depending on the type of joke, some of the fields are blank. If the joke type is single, SetUp and Delivery fields are empty.

Let’s fix that with if/else statement inside the template.

var templ = `{{range .}}-------------------------
Category: {{.Category}}
Type: {{.Type}}
{{if .Joke}}Joke: {{.Joke}} {{else}}SetUp: {{.SetUp}}
Delivery: {{.Delivery}} {{end}}
ID: {{.ID}}
Safe: {{.Safe}}
{{end}}`

It’s simple, if the object has Joke field, render it. Else, render SetUp & Delivery.