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模板是一個文字檔,定義了一個輸出檔的結構或者排版,使用定位符號表示,當模板被繪製時,資料將插入到何處(在Express,模板被稱為視圖)。

Express 模板選擇

Express 可以與許多不同的模板渲染引擎一起使用。在本教程中,我們使用 Pug(以前稱為 Jade)作為模板。這是最流行的 Node 模板語言,並且官方將自身描述為 “用於編寫 HTML,語法乾淨且空格敏感,受 Haml 影響很大”。

不同的模板語言使用不同的方法,來定義佈局和標記數據的佔位符 — 一些使用 HTML 來定義佈局,而另一些則使用可以編譯為 HTML 的不同標記格式。 Pug 是第二種類型;它使用 HTML 的表示形式,其中任何行中的第一個單詞,通常表示HTML元素,後續行中的縮進,用於表示嵌套在這些元素中的任何內容。結果是一個頁面定義直接轉換為 HTML,但可以說更簡潔,更容易閱讀。

Note: The downside of using Pug is that it is sensitive to indentation and whitespace (if you add an extra space in the wrong place you may get an unhelpful error code). However once you have your templates in place, they are very easy to read and maintain.

Template configuration

The LocalLibrary was configured to use Pug when we created the skeleton website. You should see the pug module included as a dependency in the website's package.json file, and the following configuration settings in the app.js file. The settings tell us that we're using pug as the view engine, and that Express should search for templates in the /views subdirectory.

// View engine setup.
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'pug');

If you look in the views directory you will see the .pug files for the project's default views. These include the view for the home page (index.pug) and base template (layout.pug) that we will need to replace with our own content.

/express-locallibrary-tutorial  //the project root
  /views
    error.pug
    index.pug
    layout.pug

Template syntax

The example template file below shows off many of Pug's most useful features.

The first thing to notice is that the file maps the structure of a typical HTML file, with the first word in (almost) every line being an HTML element, and indentation being used to indicate nested elements. So for example, the body element is inside an html element, and paragraph elements (p) are within the body element, etc. Non-nested elements (e.g. individual paragraphs) are on separate lines.

doctype html
html(lang="en")
  head
    title= title
    script(type='text/javascript').
  body
    h1= title

    p This is a line with #[em some emphasis] and #[strong strong text] markup.
    p This line has un-escaped data: !{'<em> is emphasised</em>'} and escaped data: #{'<em> is not emphasised</em>'}. 
      | This line follows on.
    p= 'Evaluated and <em>escaped expression</em>:' + title

    <!-- You can add HTML comments directly -->
    // You can add single line JavaScript comments and they are generated to HTML comments
    //- Introducing a single line JavaScript comment with "//-" ensures the comment isn't rendered to HTML 
    
    p A line with a link 
      a(href='/catalog/authors') Some link text
      |  and some extra text.
    
    #container.col
      if title
        p A variable named "title" exists.
      else
        p A variable named "title" does not exist.
      p.
        Pug is a terse and simple template language with a
        strong focus on performance and powerful features.
        
    h2 Generate a list
        
    ul
      each val in [1, 2, 3, 4, 5]
        li= val

Element attributes are defined in parentheses after their associated element. Inside the parentheses, the attributes are defined in comma- or whitespace- separated lists of the pairs of attribute names and attribute values, for example:

  • script(type='text/javascript'), link(rel='stylesheet', href='/stylesheets/style.css')
  • meta(name='viewport' content='width=device-width initial-scale=1')

The values of all attributes are escaped (e.g. characters like ">" are converted to their HTML code equivalents like "&gt;") to prevent injection of JavaScript/cross-site scripting attacks.

If a tag is followed by the equals sign, the following text is treated as a JavaScript expression. So for example, in the first line below, the content of the h1 tag will be variable title (either defined in the file or passed into the template from Express). In the second line the paragraph content is a text string concatented with the title variable. In both cases the default behaviour is to escape the line.

h1= title 
p= 'Evaluated and <em>escaped expression</em>:' + title

If there is no equals symbol after the tag then the content is treated as plain text. Within the plain text you can insert escaped and unescaped data using the #{} and !{} syntax, as shown below. You can also add raw HTML within the plain text.

p This is a line with #[em some emphasis] and #[strong strong text] markup. 
p This line has an un-escaped string: !{'<em> is emphasised</em>'}, an escaped string: #{'<em> is not emphasised</em>'}, and escaped variables: #{title}.

Tip: You will almost always want to escape data from users (via the #{} syntax). Data that can be trusted (e.g. generated counts of records, etc.) may be displayed without escaping the values.

You can use the pipe ('|') character at the beginning of a line to indicate "plain text". For example, the additional text shown below will be displayed on the same line as the preceding anchor, but will not be linked.

a(href='http://someurl/') Link text
| Plain text

Pug allows you to perform conditional operations using if, else , else if and unless—for example:

if title
  p A variable named "title" exists
else
  p A variable named "title" does not exist

You can also perform loop/iteration operations using each-in or while syntax. In the code fragment below we've looped through an array to display a list of variables (note the use of the 'li=' to evaluate the "val" as a variable below. The value you iterate across can also be passed into the template as a variable!

ul
  each val in [1, 2, 3, 4, 5]
    li= val

The syntax also supports comments (that can be rendered in the output—or not—as you choose), mixins to create reusable blocks of code, case statements, and many other features. For more detailed information see The Pug docs.

Extending templates

Across a site, it is usual for all pages to have a common structure, including standard HTML markup for the head, footer, navigation, etc. Rather than forcing developers to duplicate this "boilerplate" in every page, Pug allows you to declare a base template and then extend it, replacing just the bits that are different for each specific page.

For example, the base template layout.pug created in our skeleton project looks like this:

doctype html
html
  head
    title= title
    link(rel='stylesheet', href='/stylesheets/style.css')
  body
    block content

The block tag is used to mark up sections of content that may be replaced in a derived template (if the block is not redefined then its implementation in the base class is used).

The default index.pug (created for our skeleton project) shows how we override the base template. The extends tag identifies the base template to use, and then we use block section_name to indicate the new content of the section that we will override.

extends layout

block content
  h1= title
  p Welcome to #{title}

Next steps

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