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README.md

JavaScript lodash Templates

Objectives

  1. Explain how to use lodash templates
  2. Practice using lodash templates to organize JavaScript interactions
  3. Trigger changes in the DOM in response to user actions

Introduction

Dynamically adding and manipulating chunks of the DOM based on data can be a tedious, error-prone activity when we try to manually create HTML strings out of data, like this:

var commentNode = document.getElementById("comments");
commentNode.innerHTML = "<div class='some-class'>This is a comment and
it's real long</div><p>" + someDynamicValue + "</p><p>" + someOtherValue
+ "</p>";

The better way is to use a template that provides a way to use dynamically add data on a set HTML structure, preventing us from having to type out these monster strings of data and markup.

We could build out our own template for each section of our blog, creating a new function for each part, and that would work, but there are tools out there to make this task nice and easy for us.

lodash

Lodash is a JavaScript library that provides a lot of utilities for working with arrays, objects, and strings, including a template library which is what we'll be working with today.

Let's look at the earlier example:

commentNode.innerHTML = "<div class='some-class'>This is a comment and
it's real long</div><p>" + someDynamicValue + "</p><p>" + someOtherValue
+ "</p>"

The <div class='some-class'>... string is our template. It's not very pretty, but it's ours. We load data into the template by closing the string and concatenating variables then continuing the template string. If you're anything like me, you've forgotten to type at least one of the plus signs nearly every time you've done any long string concatenation, so, bare minimum, our templating tool should fix this problem.

Basic Templating With Interpolation

Let's wire up a basic comments form to our blog post. Starting with the markup in index.html, we'll add a simple form, and a placeholder for the comments to be displayed.

<article>
  <header><h2>The Results are In!</h2></header>
  <p>
    After careful consideration and a generous grant from NASA, our team has determined that a woodchuck could chuck five wood, with a standard deviation of +/- .37 wood.
  </p>
  <footer>posted by Chuck Wooden</footer>
  <form onsubmit="postComment();return false;">
    Name: <input type="text" id="commenterName"><br>
    Comment: <input type="textarea" id="commentText"><br>
    <input type="submit">
  </form>
  <div id="comments"></div>
</article>

So we have a basic form that will submit with a postComment() function. Let's add the basic bits of that to our index.js.

function postComment() {
  var commenter = document.getElementById("commenterName").value;
  var comment = document.getElementById("commentText").value;
  //insert comment into "comments" div in this format:
  //<div class="comment"><p>comment</p><p>Posted By: <span class="commenter">commenter</span></p></div>
}

We know the format of the template we want, but instead of adding it using string concatenation, we're going to use the lodash template function.

Top-tip: By default, we access lodash functions through the underscore, which is a variable that holds the root object, like _.template(). This is similar to accessing jQuery functions via the $ variable. This is because lodash was originally a fork of the Underscore.js library.

We'll start by building a template string as we have in the past, but instead of concatenation, we'll be using interpolation, which is a way to replace text in strings using special delimeters.

In lodash, we can interpolate strings with the <%= %> delimeter, so let's try it out.

function postComment() {
  var commenter = document.getElementById("commenterName").value;
  var comment = document.getElementById("commentText").value;
  //insert comment into "comments" div in this format:
  //<div class="comment"><p>comment</p><p>Posted By: <span class="commenter">commenter</span></p></div>

  var commentTemplate = '<div class="comment"><p><%= comment %></p><p>Posted By: <span class="commenter"><%= commenter %></span></p></div>';
}

We're creating a new variable called commentTemplate that will hold our template string. We've wrapped our two variables inside the interpolation delimiters, with <%= comment %> and <%= commenter %>.

The next step is to use the template() function of lodash to turn this template string into a function that we can call with a JSON argument for the variable values.

function postComment() {
  var commenter = document.getElementById("commenterName").value;
  var comment = document.getElementById("commentText").value;
  //insert comment into "comments" div in this format:
  //<div class="comment"><p>comment</p><p>Posted By: <span class="commenter">commenter</span></p></div>

  //create template string
  var commentTemplate = '<div class="comment"><p><%= comment %></p><p>Posted By: <span class="commenter"><%= commenter %></span></p></div>';
  //create template function
  var templateFn = _.template(commentTemplate);
}

We create a function here by executing _.template(), but we don't execute it now. Why?

One of the reasons to use templating is to be able to reuse the templates. By creating a function and holding it in a variable, we can execute the function over and over with different data, rather than having to rebuild the template function every time.

Now that we have our function, we'll need to execute it with our form data as a regular JSON object: { 'comment': value, 'commenter': value }. Note that the keys for the object match the variable names inside our interpolation delimiters.

Executing the function will return a string, which we can then append to our comments div. Let's finish it out.

function postComment() {
  var commenter = document.getElementById("commenterName").value;
  var comment = document.getElementById("commentText").value;
  //insert comment into "comments" div in this format:
  //<div class="comment"><p>comment</p><p>Posted By: <span class="commenter">commenter</span></p></div>

  //create template string
  var commentTemplate = '<div class="comment"><p><%= comment %></p><p>Posted By: <span class="commenter"><%= commenter %></span></p></div>';
  //create template function
  var templateFn = _.template(commentTemplate);

  var commentsDiv = document.getElementById("comments");

  //execute template function with JSON object for the interpolated values
  var templateHTML = templateFn({ 'comment': comment, 'commenter': commenter });

  //append rather than replace!
  commentsDiv.innerHTML += templateHTML;
}

Reload your index.html and see it in action!

Okay, this is all fine and good, and we aren't concatenating strings anymore, but we're still building out a huge string for the template, and this is a pretty simple template. What happens when we have a lot of markup to work with?

HTML Templates in JavaScript

Putting our templates in a string isn't great for a couple of reasons. We don't get the human-readable, indented structure of regular HTML. We don't get the syntax highlighting on the tags that our editor provides. Adding or changing HTML inside a string is difficult and prone to error. What we need is a way to use a regular chunk of HTML as our template.

Fortunately, lodash gives us a great way to do this. We can use a special <script> tag with the content-type of text/x-lodash-template and embed HTML directly inside. Let's redo our comment template.

This time, we're going to put this in the bottom of our index.html like it was any other JavaScript block.

<!-- index.html, place before end body tag -->
<body...>
  <article>...</article>
  <script id="comment-template" type="text/x-lodash-template">
    <div class="comment">
      <div class="comment-body"><%= comment %></div>
      <div class="commenter">
        <p>
          <span class="posted-by">Posted By: </span>
          <%= commenter %>
        </p>
      </div>
    </div>
  </script>
</body>

Inside that <script id="comment-template"> tag we just have mostly regular HTML! The only difference is, since we're inside a script tag, we also can use the interpolation delimiters from lodash to mark where our dynamic data is.

Okay, we've made a significant change here, so surely there's a lot of work require to wire this up, right?

// index.js
function postComment() {
  var commenter = document.getElementById("commenterName").value;
  var comment = document.getElementById("commentText").value;

  //insert comment into "comments" div in this format:
  //<div class="comment"><p>comment</p><p>Posted By: <span class="commenter">commenter</span></p></div>

  //create template string - THIS IS THE ONLY LINE WE HAVE TO CHANGE
  //var commentTemplate = '<div class="comment"><p><%= comment %></p><p>Posted By: <span class="commenter"><%= commenter %></span></p></div>';
  var commentTemplate = document.getElementById("comment-template").innerHTML;

  //create template function
  var templateFn = _.template(commentTemplate);

  var commentsDiv = document.getElementById("comments");

  //execute template function with JSON object for the interpolated values
  var templateHTML = templateFn({ 'comment': comment, 'commenter': commenter });

  //append rather than replace!
  commentsDiv.innerHTML += templateHTML;
}

Nope! We just had to change the one line that defines the source string for the template. Remember that HTML is just a string, and we can read the innerHTML property of any element. Since we gave the script tag an id, we know how to query it, read its contents, and use that as our template string.

Let's reload and give it a shot. Works great, right?

Since lodash expects a string argument for template(), we just fed it a different template string with the same interpolated values and everything else works just fine. Master hackers over here.

mr robot

Using this technique, you can build out one or more HTML templates inside script tags, taking advantage of the ease of editing HTML in its normal form, then take the innerHTML of those script tags and use it to build lodash templates. No more giant strings!

Other Template Delimiters

We've seen how to interpolate text into our templates, but what if we want to prevent HTML tags from being evaluated? Or execute some JavaScript code on an interpolated value?

Lodash gives us what we need here as well, allowing us to use different delimiters to interpolate different things.

Escaping HTML Data

Let's say we don't want to allow our commenters to use HTML markup in their comments. Right now, if you tried posting a comment like <p>hi</p><p>everyone</p><h1>this is my comment</h1>, our form would let you. But of course, like everything else, trolls ruin this and find ways to break our blog style with their markup, so we want to change our comment field to not execute HTML tags.

Luckily, it's as simple as changing the delimiter from interpolate (<%= %>) to escape (<%- %>). Let's try it:

<script id="comment-template" type="text/x-lodash-template">
  <div class="comment">
    <div class="comment-body"><%- comment %></div>
    <div class="commenter">
      <p>
        <span class="posted-by">Posted By: </span>
        <%- commenter %>
      </p>
    </div>
  </div>
</script>

A quick refresh and try to post that same comment again, and we'll see that our HTML has been escaped and the tags display harmlessly on the page.

Executing Code

Let's say we want to execute some non-printing JavaScript code in our template. We can use the <% %>, or eval delimiter.

For instance, we want to display the commenter's name, but if they accidentally left it blank, we want to display "Anonymous". We can use the eval delimiter to execute an if...else statement.

<div class="commenter">
  <p>
    <span class="posted-by">Posted By: </span>
    <% if(commenter !== '') { %>
    <%= commenter %>
    <% } else { %>
    Anonymous
    <% } %>
  </p>
</div>

Top-tip: The eval delimiter is for executing non-printing code, that is, code that won't display a result directly on the page, such as if...then blocks or for loops. If you wanted to display the results of a function, say, Date.now() on the page, you would use the regular interpolation delimiter, like <%= Date.now() %>.

Summary

In this lesson we learned how to use lodash and the template() function to create templates and dynamically add templated data to our DOM. We saw that we can use a script tag to hold complex HTML templates rather than building our templates inside string variables.

Finally, we learned how to use the different delimiters to display text, escape HTML, and execute code for more powerful templates.

Resources

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