Check-Driving HTML Templates

After a decade or extra the place Single-Web page-Purposes generated by
JavaScript frameworks have
change into the norm, we see that server-side rendered HTML is changing into
standard once more, additionally because of libraries reminiscent of HTMX or Turbo. Writing a wealthy internet UI in a
historically server-side language like Go or Java is no longer simply potential,
however a really engaging proposition.

We then face the issue of how one can write automated exams for the HTML
components of our internet functions. Whereas the JavaScript world has developed highly effective and refined methods to check the UI,
ranging in measurement from unit-level to integration to end-to-end, in different
languages we do not need such a richness of instruments obtainable.

When writing an internet utility in Go or Java, HTML is usually generated
by way of templates, which comprise small fragments of logic. It’s definitely
potential to check them not directly by way of end-to-end exams, however these exams
are gradual and costly.

We will as an alternative write unit exams that use CSS selectors to probe the
presence and proper content material of particular HTML parts inside a doc.
Parameterizing these exams makes it simple so as to add new exams and to obviously
point out what particulars every take a look at is verifying. This method works with any
language that has entry to an HTML parsing library that helps CSS
selectors; examples are offered in Go and Java.

Stage 1: checking for sound HTML

The primary factor we wish to examine is that the HTML we produce is
principally sound. I do not imply to examine that HTML is legitimate in keeping with the
W3C; it might be cool to do it, nevertheless it’s higher to start out with a lot less complicated and quicker checks.
As an illustration, we wish our exams to
break if the template generates one thing like

<div>foo</p>

Let’s have a look at how one can do it in levels: we begin with the next take a look at that
tries to compile the template. In Go we use the usual html/template package deal.

  func Test_wellFormedHtml(t *testing.T) {
    templ := template.Should(template.ParseFiles("index.tmpl"))
    _ = templ
  }

In Java, we use jmustache
as a result of it is quite simple to make use of; Freemarker or
Velocity are different widespread decisions.

Java

  @Check
  void indexIsSoundHtml() {
      var template = Mustache.compiler().compile(
              new InputStreamReader(
                      getClass().getResourceAsStream("/index.tmpl")));
  }

If we run this take a look at, it can fail, as a result of the index.tmpl file does
not exist. So we create it, with the above damaged HTML. Now the take a look at ought to move.

Then we create a mannequin for the template to make use of. The applying manages a todo-list, and
we are able to create a minimal mannequin for demonstration functions.

  func Test_wellFormedHtml(t *testing.T) {
    templ := template.Should(template.ParseFiles("index.tmpl"))
    mannequin := todo.NewList()
    _ = templ
    _ = mannequin
  }

Java

  @Check
  void indexIsSoundHtml() {
      var template = Mustache.compiler().compile(
              new InputStreamReader(
                      getClass().getResourceAsStream("/index.tmpl")));
      var mannequin = new TodoList();
  }

Now we render the template, saving the leads to a bytes buffer (Go) or as a String (Java).

  func Test_wellFormedHtml(t *testing.T) {
    templ := template.Should(template.ParseFiles("index.tmpl"))
    mannequin := todo.NewList()
    var buf bytes.Buffer
    err := templ.Execute(&buf, mannequin)
    if err != nil {
      panic(err)
    }
  }

Java

  @Check
  void indexIsSoundHtml() {
      var template = Mustache.compiler().compile(
              new InputStreamReader(
                      getClass().getResourceAsStream("/index.tmpl")));
      var mannequin = new TodoList();
  
      var html = template.execute(mannequin);
  }

At this level, we wish to parse the HTML and we count on to see an
error, as a result of in our damaged HTML there’s a div aspect that
is closed by a p aspect. There’s an HTML parser within the Go
commonplace library, however it’s too lenient: if we run it on our damaged HTML, we do not get an
error. Fortunately, the Go commonplace library additionally has an XML parser that may be
configured to parse HTML (because of this Stack Overflow reply)

  func Test_wellFormedHtml(t *testing.T) {
    templ := template.Should(template.ParseFiles("index.tmpl"))
    mannequin := todo.NewList()
    
    // render the template right into a buffer
    var buf bytes.Buffer
    err := templ.Execute(&buf, mannequin)
    if err != nil {
      panic(err)
    }
  
    // examine that the template may be parsed as (lenient) XML
    decoder := xml.NewDecoder(bytes.NewReader(buf.Bytes()))
    decoder.Strict = false
    decoder.AutoClose = xml.HTMLAutoClose
    decoder.Entity = xml.HTMLEntity
    for {
      _, err := decoder.Token()
      swap err {
      case io.EOF:
        return // We're performed, it is legitimate!
      case nil:
        // do nothing
      default:
        t.Fatalf("Error parsing html: %s", err)
      }
    }
  }

	function	CSS selector
①	All of the gadgets	ul.todo-list li
②	Accomplished gadgets	ul.todo-list li.accomplished
⓷	Objects left	span.todo-count
④	Highlighted navigation hyperlink	ul.filters a.chosen
⑤	Clear accomplished button	button.clear-completed

Check-Driving HTML Templates

Stage 1: checking for sound HTML

Stage 2: testing HTML construction

What else ought to we take a look at?

Testing HTML content material

Check each content material and soundness on the identical time

The second take a look at

Make it simple so as to add new exams

Making the desk extra expressive, on the expense of the take a look at code

Stage 3: testing HTML behaviour

Loading the web page within the Playwright browser

Figuring out todo gadgets

Clicking on a todo merchandise

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