v1.1.1Published 2 years ago


HTMLjs is a small library for expressing HTML trees with a concise syntax. It is used to render content in Blaze and to represent templates during compilation.


  UL({id: 'mylist'},
     LI({'class': 'item'}, "Hello ", B("world"), "!"),
     LI({'class': 'item'}, "Goodbye, world")))
<ul id="mylist">
  <li class="item">Hello <b>world</b>!</li>
  <li class="item">Goodbye, world</li>

The functions UL, LI, and B are constructors which return instances of HTML.Tag. These tag objects can then be converted to an HTML string or directly into DOM nodes.

The flexible structure of HTMLjs allows different kinds of Blaze directives to be embedded in the tree. HTMLjs does not know about these directives, which are considered "foreign objects."

Built-in Types

The following types are built into HTMLjs. Built-in methods like HTML.toHTML require a tree consisting only of these types.

  • null, undefined - Render to nothing.

  • boolean, number - Render to the string form of the boolean or number.

  • string - Renders to a text node (or part of an attribute value). All characters are safe, and no HTML injection is possible. The string "<a>" renders &lt;a> in HTML, and document.createTextNode("<a>") in DOM.

  • Array - Renders to its elements in order. An array may be empty. Arrays are detected using HTML.isArray(...).

  • HTML.Tag - Renders to an HTML element (including start tag, contents, and end tag).

  • HTML.CharRef({html: ..., str: ...}) - Renders to a character reference (such as &nbsp) when generating HTML.

  • HTML.Comment(text) - Renders to an HTML comment.

  • HTML.Raw(html) - Renders to a string of HTML to include verbatim.

The new keyword is not required before constructors of HTML object types.

All objects and arrays should be considered immutable. Instance properties are public, but they should only be read, not written. Arrays should not be spliced in place. This convention allows for clean patterns of processing and transforming HTMLjs trees.


An HTML.Tag is created using a tag-specific constructor, like HTML.P for a <p> tag or HTML.INPUT for an <input> tag. The resulting object is instanceof HTML.Tag. (The HTML.Tag constructor should not be called directly.)

Tag constructors take an optional attributes dictionary followed by zero or more children:


HTML.DIV(HTML.P("First paragraph"),
         HTML.P("Second paragraph"))

HTML.INPUT({type: "text"})

HTML.SPAN({'class': "foo"}, "Some text")

Instance properties

Tags have the following properties:

  • tagName - The tag name in lowercase (or camelCase)
  • children - An array of children (always present)
  • attrs - An attributes dictionary, null, or an array (see below)

Special forms of attributes

The attributes of a Tag may be an array of dictionaries. In order for a tag constructor to recognize an array as the attributes argument, it must be written as HTML.Attrs(attrs1, attrs2, ...), as in this example:

var extraAttrs = {'class': "container"};

var div = HTML.DIV(HTML.Attrs({id: "main"}, extraAttrs),
                   "This is the content.");

div.attrs // => [{id: "main"}, {'class': "container"}]

HTML.Attrs may also be used to pass a foreign object in place of an attributes dictionary of a tag.

Normalized Case for Tag Names

The tagName field is always in "normalized case," which is the official case for that particular element name (usually lowercase). For example, HTML.DIV().tagName is "div". For some elements used in inline SVG graphics, the correct case is "camelCase." For example, there is an element named clipPath.

Web browsers have a confusing policy about case. They perform case normalization when parsing HTML, but not when creating SVG elements at runtime; the correct case is required.

Therefore, in order to avoid ever having to normalize case at runtime, the policy of HTMLjs is to put the burden on the caller of functions like HTML.ensureTag -- for example, a template engine -- of supplying correct normalized case.

Briefly put, normlized case is usually lowercase, except for certain elements where it is camelCase.

Known Elements

HTMLjs comes preloaded with constructors for all "known" HTML and SVG elements. You can use HTML.P, HTML.DIV, and so on out of the box. If you want to create a tag like <foo> for some reason, you have to tell HTMLjs to create the HTML.FOO constructor for you using HTML.ensureTag or HTML.getTag.

HTMLjs's lists of known elements are public because they are useful to other packages that provide additional functions not found here, like functions for normalizing case.

Foreign objects

Arbitrary objects are allowed in HTMLjs trees, which is useful for adapting HTMLjs to a wide variety of uses. Such objects are called foreign objects.

The one restriction on foreign objects is that they must be instances of a class -- so-called "constructed objects" (see HTML.isConstructedObject) -- so that they can be distinguished from the vanilla JS objects that represent attributes dictionaries when constructing Tags.

Functions are also considered foreign objects.


  • tagName - A string in normalized case

Creates a tag constructor for tagName, assigns it to the HTML namespace object, and returns it.

For example, HTML.getTag("p") returns HTML.P. HTML.getTag("foo") will create and return HTML.FOO.

It's very important that tagName be in normalized case, or else an incorrect tag constructor will be registered and used henceforth.


  • tagName - A string in normalized case

Ensures that a tag constructor (like HTML.FOO) exists for a tag name (like "foo"), creating it if necessary. Like HTML.getTag but does not return the tag constructor.


  • tagName - A string in normalized case

Returns whether a particular tag is guaranteed to be available on the HTML object (under the name returned by HTML.getSymbolName).

Useful for code generators.


  • tagName - A string in normalized case

Returns the name of the all-caps constructor (like "FOO") for a tag name in normalized case (like "foo").

In addition to converting tagName to all-caps, hyphens (-) in tag names are converted to underscores (_).

Useful for code generators.


An array of all known HTML5 and SVG element names in normalized case.


An array of all known SVG element names in normalized case.

The "a" element is not included because it is primarily a non-SVG element.


An array of all "void" element names in normalized case. Void elements are elements with a start tag and no end tag, such as BR, HR, IMG, and INPUT.

The HTML spec defines a closed class of void elements.


  • tagName - A string in normalized case

Returns whether tagName is a known HTML5 or SVG element.


  • tagName - A string in normalized case

Returns whether tagName is the name of a known SVG element.


  • tagName - A string in normalized case

Returns whether tagName is the name of a void element.

HTML.CharRef({html: ..., str: ...})

Represents a character reference like &nbsp;.

A CharRef is not required for escaping special characters like <, which are automatically escaped by HTMLjs. For example, HTML.toHTML("<") is "&lt;". Also, now that browsers speak Unicode, non-ASCII characters typically do not need to be expressed as character references either. The purpose of CharRef is offer control over the generated HTML, allowing template engines to preserve any character references that they come across.

Constructing a CharRef requires two strings, the uninterpreted "HTML" form and the interpreted "string" form. Both are required to be present, and it is up to the caller to make sure the information is accurate.

Examples of valid CharRefs:

  • HTML.CharRef({html: '&amp;', str: '&'})
  • `HTML.CharRef({html: ' ', str: '\u00A0'})

Instance properties: .html, .str


  • value - String

Represents an HTML Comment. For example, HTML.Comment("foo") represents the comment <!--foo-->.

The value string should not contain two consecutive hyphens (--) or start or end with a hyphen. If it does, the offending hyphens will be stripped before generating HTML.

Instance properties: value


  • value - String

Represents HTML code to be inserted verbatim. value must consist of a valid, complete fragment of HTML, with all tags closed and all required end tags present.

No security checks are performed, and no special characters are escaped. Raw should not be used if there are other ways of accomplishing the same result. HTML supplied by an application user should not be rendered unless the user is trusted, and even then, there could be strange results if required end tags are missing.

Instance properties: value


Returns whether x is considered an array for the purposes of HTMLjs. An array is an object created using [...] or new Array.

This function is provided because there are several common ways to determine whether an object should be treated as an array in JavaScript.


Returns whether x is a "constructed object," which is (loosely speaking) an object that was created with new Foo (for some Foo) rather than with {...} (a vanilla object). Vanilla objects are used as attribute dictionaries when constructing tags, while constructed objects are used as children.

For example, in HTML.DIV({id:"foo"}), {id:"foo"} is a vanilla object. In HTML.DIV(HTML.SPAN("text")), the HTML.SPAN is a constructed object.

A simple constructed object can be created as follows:

var Foo = function () {};
var x = new Foo; // x is a constructed object

In particular, the test is that x is an object and x.constructor is set, but on a prototype of the object, not the object itself. The above example works because JavaScript sets Foo.prototype.constructor = Foo when you create a function Foo.


Returns true if content is null, undefined, an empty array, or an array of only "nully" elements.


Returns whether name is a valid name for an attribute of an HTML tag or element. name must:

  • Start with :, _, A-Z or a-z
  • Consist only of those characters plus -, ., and 0-9.

Discussion: The HTML spec and the DOM API (setAttribute) have different definitions of what characters are legal in an attribute. The HTML parser is extremely permissive (allowing, for example, <a %=%>), while setAttribute seems to use something like the XML grammar for names (and throws an error if a name is invalid, making that attribute unsettable). If we knew exactly what grammar browsers used for setAttribute, we could include various Unicode ranges in what's legal. For now, we allow ASCII chars that are known to be valid XML, valid HTML, and settable via setAttribute.

See and


If attrs is an array, the attribute dictionaries in the array are combined into a single attributes dictionary, which is returned. Any "nully" attribute values (see HTML.isNully) are ignored in the process. If attrs is a single attribute dictionary, a copy is returned with any nully attributes removed. If attrs is equal to null or an empty array, null is returned.

Attribute dictionaries are combined by assigning the name/value pairs in array order, with later values overwriting previous values.

attrs must not contain any foreign objects.


  • content - any HTMLjs content

Returns a string of HTML generated from content.

For example:

HTML.toHTML(HTML.HR()) // => "<hr>"

Foreign objects are not allowed in content. To generate HTML containing foreign objects, create a subclass of HTML.ToHTMLVisitor and override visitObject.

HTML.toText(content, textMode)

  • content - any HTMLjs content
  • textMode - the type of text to generate; one of HTML.TEXTMODE.STRING, HTML.TEXTMODE.RCDATA, or HTML.TEXTMODE.ATTRIBUTE

Generating HTML or DOM from HTMLjs content requires generating text for attribute values and for the contents of TEXTAREA elements, among others. The input content may contain strings, arrays, booleans, numbers, nulls, and CharRefs. Behavior on other types is undefined.

The required textMode argument specifies the type of text to generate:

  • HTML.TEXTMODE.STRING - a string with no special escaping or encoding performed, suitable for passing to setAttribute or document.createTextNode.
  • HTML.TEXTMODE.RCDATA - a string with < and & encoded as character references (and CharRefs included in their "HTML" form), suitable for including in a string of HTML
  • HTML.TEXTMODE.ATTRIBUTE - a string with " and & encoded as character references (and CharRefs included in their "HTML" form), suitable for including in an HTML attribute value surrounded by double quotes