2012 June « coolmuse

… or what to do when there’s no DataView.

updated: corrected the output byte order in formatIEEE64().

i recently needed to be able to serialize a JavaScript Number to a 64-bit IEEE 754 floating point format.

there were two main challenges to overcome:

there is no universal way to get direct access to the bits that make up the JavaScript Number. DataView will change this, but it’s not available on IE9. So I needed to find a way to decode the JavaScript number into its constituent sign, exponent, and mantissa, with proper handling of denormalized values.
the mantissa, once calculated, is a 52-bit value, so we can’t use JavaScript’s bitwise operators to access the bits of the mantissa. The bitwise operators all coerce their arguments to 32-bit values.

jDataView is a library that has the ability to read 64-bit IEEE values from a buffer, and it has been recently updated to also support writing 64-bit IEEE values.

here is my solution to the problem until DataView becomes widely available.

… or how to stand on the shoulders of giants.

i’m sure that most of you who are interested in this post will be somewhat familiar with this John Resig post of 2008, where he outlines a simple process for creating object (class?) hierarchies in JavaScript.

as a relative newcomer to JavaScript after 20 years of C/C++/C# development, the last few months have provided a fun challenge. I cut my JavaScript teeth developing the HTML5 ebook From Blue To Red, and that experience provided an excellent opportunity to match my preconceptions of strongly-typed classical inheritance to the strange new world of JavaScript prototypes.

i’m now working on another project which is modeled well with a couple of shallow class hierarchies, and the JavaScript components cannot depend on any other libraries during a bootstrap phase. Resig’s pattern provided an excellent starting point for me, but I ran into trouble when I started using ECMAScript 5 getter and setter properties, which were actually being invoked on reference when copying properties from the object literal to the class prototype.

here then is a version of Resig’s Simple JavaScript Inheritance, revisited and revised for ECMAScript 5 and compliance with “use strict”.

coolmuse

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getting the exponent and mantissa from a JavaScript number

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