Before you decide to use this library, be aware that OpenType.js has become better at doing what you were probably thinking of using Font.js for - I still think JavaScript needs a Font object, but at this point rewriting it from scratch so that it's backed by OpenType.js is far more sensible than using Font.js in its current form. So: head on over there and discover an amazing library.

With that said....

Font.js adds new Font() functionality to the JavaScript toolbox, akin to how you use new Image() for images. It adds an onload event hook so that you don't deploy a font resource on your webpage before it's actually ready for use, as browsers can tell you when a font has downloaded, but not when it has been parsed and made ready for on-page rendering. This object can. It can also do this for system fonts, so you can also use Font.js to detect whether a specific system font is installed at the client, for fallbacks and failsafes.

The API is pretty straight forward:

• constructor

  var font = new Font();

• load event handler

  font.onload = function() { /* your code here */ };

• error event handler

  font.onerror = function(error_message) { /* your code here */ };

• name assignment

  font.fontFamily = "name goes here";

• font loading

  • remote fonts:

    font.src = "http://your.font.location/here.ttf";

  (!) This line will kick off font loading and will make the font available on-page (if a remote font was requested).

  • system fonts / already @font-face loaded fonts:

    font.src = font.fontFamily;

  (!) Note that this also works for google webfont and typekit fonts that have been loaded through an HTML stylesheet. Any font that is available by name, rather than by font file, can be loaded using the above "src=fontFamily" solution.

• DOM removal

  document.head.removeChild(font.toStyleNode());

  (!) this only applies to fonts loaded from a remote resource. System fonts do not have an associated style node.

• DOM (re)insertion

  document.head.appendChild(font.toStyleNode());

  (!) This is only required if you removed the font from the page, as the font is added to the DOM for use on-page during font loading already.

  (!) this only applies to fonts loaded from a remote resource. System fonts do not have an associated style node.

• font.metrics.quadsize

  The font-indicated number of units per em

• font.metrics.leading

  The font-indicated line height, in font units (this vaue is, often, useless)

• font.metrics.ascent

  The maximum ascent for this font, as a ratio of the fontsize

• font.metrics.decent

  The maximum descent for this font, as a ratio of the fontsize

• font.metrics.weightclass

  The font-indicated weight class

(!) As system font files cannot be inspected, they do not have an associated font.metrics object. Instead, font.metrics is simply "false".

• font.measureText(string, size)

  Compute the metrics for a particular string, with this font applied at the specific font size in pixels

• font.measureText(...).width

  the width of the string in pixels, using this font at the specified font size

• font.measureText(...).fontsize

  the specified font size

• font.measureText(...).height

  the height of the string. This may differ from the font size

• font.measureText(...).leading

  the actual line spacing for this font based on ten lines.

• font.measureText(...).ascent

  the ascent above the baseline for this string

• font.measureText(...).descent

  the descent below the baseline for this string

• font.measureText(...).bounds

  An object {xmin:, ymin:, xmax:, ymax:} indicating the string's bounding box.

When font.src is set, the whole shebang kicks in, just like for new Image(), so make sure to define your onload() handler BEFORE setting the "src" property, or your handler may not get called.

A demonstrator of this object can be found at:

• http://pomax.nihongoresources.com/pages/Font.js

Font.js is compatible with all browsers that support canvas and Object.defineProperty -- This includes all versions of Firefox, Chrome, Opera, IE and Safari that were 'current' (Firefox 9, Chrome 16, Opera 11.6, IE9, Safari 5.1) at the time Font.js was released.

Note: Font.js will not work on IE8 or below due to the lack of Object.defineProperty

This code is (c) Mike "Pomax" Kamermans, 2012, but licensed under the MIT ("expat" flavour) license.