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This library is a utility belt to make AST transforms and shield users as much as possible from the nuances of the AST, as it is still private API.



This class is the main interface users should use. It gives you a nice declarative way of defining complex transforms from curly components to HTMLElements that resembles Ember.Component in its API.

Basic API

The basic usage is simple:

let component = new BuildTimeComponent(node);

This alone mimics the behaviour of Ember.Component in some ways:

  • Generates a div element
  • Binds the class= attribute received on invocation to the class on the element.

It also accepts an object with options to configure it, much like Ember.Component.extend(opts):

let component = new BuildTimeComponent(node, {
  tagName: 'span',
  classNames: ['my-component'],
  classNameBindings: ['isActive:is-active:is-disabled'],
  attributeBindings: ['title', 'ariaLabel:aria-label'],
  isActive: true

This will be smart enough to generate the appropriate transformations:

Original Transformed
{{my-component class="simple-example"}} <span class="my-component is-active simple-example"></span>
{{my-component class=someClass}} <span class="my-component {{someClass}}"></span>
{{my-component class="simple-example" isActive=false}} <span class="my-component simple-example"></span>
{{my-component class="simple-example" isActive=isActive}} <span class="my-component {{if isActive 'is-active' 'is-disabled'}} simple-example"></span>
{{my-component class="simple-example" title="Hello" ariaLabel="World"}} <span class="my-component is-active simple-example" title="Hello" aria-label="World"></span>
{{my-component class="simple-example" title=title}} <span class="my-component is-active simple-example" title={{title}}></span>

Creating your own components

Just as you'd expect from Ember.Component, you can subclass BuildTimeComponent to configure it once and reuse it many times, all in a nice ES6 syntax. And classNames, classNameBindings and attributeBindings work as concatenated properties.

class MyComponent extends BuildTimeComponent {
  constructor(node, { tagName = 'span', isActive = true, }) {
    super(node, { tagName, isActive, });
    this.classNames = ['my-component'];
    this.classNameBindings = ['isActive:is-active:is-disabled'];
    this.attributeBindings = ['title', 'ariaLabel:aria-label'];

In the future once Class properties are implemented (Stage 3 right now) you will be able to DRY up the code above:

class MyComponent extends BuildTimeComponent {
  tagName = 'span'
  classNames = ['my-component']
  classNameBindings = ['isActive:is-active:is-disabled']
  attributeBindings = ['title', 'ariaLabel:aria-label']
  isActive = true

What about classes/attributes that cannot be expressed with simple bindings? For that, you can declare functions named <propName>Content and that function will win over runtime options, extension options or invocation options.

To clarify that, you need to understand that there is 4 ways components can get their title:

class Foo extends BuildTimeComponent {
  super(node, opts) {
    super(node, opts);
    this.title = 'Extension time title';

  titleContent() {
    return "Computed title";
let component = new Foo(node, { title: 'Initialization-time title' });
  {{my-foo title="Runtime title"}}

The precedence rules are:

  1. <propName>Content(){ } wins over everything. More on this later.
  2. In its absence, the runtime argument ({{my-foo propName="value"}}) wins.
  3. In the absence of both, the options passed when the component is instantiated (new Foo(node, { propName: 'value' })) wins.
  4. Lastly if none is provided, the default value when the class is defined is applied.

<propName>Content(){ } and how to use it

You just read above that the method <propName>Content wins over absolutely any other way the user has to provide <propName> to the component. However, typically you will compute the value of a property based on some inputs. Perhaps the runtime arguments, perhaps the init options, or perhaps all of them.

Within this method, you can access all those values:

  1. this.<propName> for values assigned with this.<propName> inside the constructor
  2. this.options.<propName> for values passed on the initialization (new Foo(node, { propName: 'value' }))
  3. this.attrs.<propName> for values passed on the template ({{my-foo propName=value}})


Unlike Ember.Component, positional params can be listed without reopening the class

class MyLink extends BuildTimeComponent {
  constructor(node, { tagName = 'span', isActive = true, }) {
    super(node, { tagName, isActive, });
    this.positionalParams = ['url', 'text'];
    this.attributeBindings = ['url:href', 'text:aria-label'];
// {{my-link "foo/bar.html" "About us"}}


Like regular components, build-time components can define their own layout, using a similar approach to the one used in ember-cli-htmlbars-inline-precompile. BuildTimeComponents are smart enough to detect if the component is invoked with or without a block, and simplify conditionals for you, and replace arguments in the templates with the equivalent values in the parent scope.

P.e, Given a component with a layout like this:

class MyComponent extends BuildTimeComponent {
  constructor(node: BuildTimeComponentNode, opts?: Partial<BuildTimeComponentOptions>) {
    super(node, opts);
        {{other-component thing=value}}
        {{#if hasBlock}}
      <strong>Other content for {{world}}</strong>

When it is invoked like this:

{{my-component world=planet value="Dog"}}

Then it compiles down to:

    {{other-component thing="Dog"}}
  <strong>Other content for {{planet}}</strong>

NOTE ON TEMPLATES: It is important to note that since BuildTimeComponents are transformed in compile time, the template of this kind of components is going to be inlined as many times as times the component is invoked. If the template is very large and the component is used very often, this can have a negative impact on the size of the application. Although this kind of repetition compresses very well with gzip, have this in mind if your component has a large template.

Other helpers

  • buildAttr(builder, attributeName, content) => AttrNode: Content can be pretty much anything. JS Strings, StringLiterals, TextNodes, PathExpression, ConcatStatements ... Just pass things down, it will do the right thing.

  • appendToContent(builder, content, dataToAppend, options) => newContent: It takes cares of the nuances of joining content together. It can be used by example to construct the content of an attribute like class from several pieces. It accepts pretty much anything. By default it adds a space between values, but that can be changed passing prependSpace: false on the options.

  • interpolateProperties(interpolation: string, { divisor = ':', skipIfMissing = true, skipIfMissingDynamic = false, callback }): A convenient method to generate interpolate values into strings. It accepts an object with options. P.e: styleContent: interpolateProperties('color: $color$; background-color: $bgColor$')


Utility belt to level-up your Ember AST transforms







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