--- a/core-javascript/equality.rst	Wed Oct 19 17:55:10 2011 +0100
+++ b/core-javascript/equality.rst	Fri Oct 21 10:48:27 2011 +0100
@@ -1,2 +1,146 @@
 Equality
 ========
+
+Equality in JavaScript can be a little odd.
+
+
+Double equals
+-------------
+
+JavScript has two operations for equality (and inequality).  One of
+them is '==' (and '!=' for inequality).  It's the oddest.
+
+
+Not transitive
+^^^^^^^^^^^^^^
+
+If *a* is equal to *b*, and if *b* is equal to *c* , then we expect
+*a* to be equal to *c*.  This is called the transitive property.
+
+
+.. code-block:: javascript
+
+
+   js> '0' == 0      // 'a' is equal to 'b'
+   true
+   js> 0 == ''      //  'b' is equal to 'c'
+   true
+   js> '0' == ''    // 'a' is not equal to 'c'
+   false
+
+
+
+Not reflexive
+^^^^^^^^^^^^^
+
+We expect *a* to be equal to *a*.  This is called the reflexive
+property.
+
+.. code-block:: javascript
+
+   js> NaN == NaN
+   false
+
+Fortunately, this seems to be the only example.
+
+
+Is symmetric
+^^^^^^^^^^^^
+
+If *a* is equal to *b* then we expect *b* to be equal to *a*.  This is
+called the symmetric property.  In JavaScript equality is symmetric.
+
+
+
+Triple equality
+---------------
+
+JavaScript also has '===' (and '!==' for inequality).
+
+
+Is transitive
+^^^^^^^^^^^^^
+
+Unlike double equals, triple equals is always transitive.
+
+.. code-block:: javascript
+
+   js> '0' === 0      // unequal
+   false
+   js> 0 === ''       // unequal
+   false
+   js> '0' === ''     // unequal
+   false
+
+
+Not reflexive
+^^^^^^^^^^^^^
+
+.. code-block:: javascript
+
+   js> NaN === NaN
+   false
+
+
+By the way, there's a thread on Facebook with subject `Time and Date
+on my wall shows NaNNaNNaN at NaN:NaN
+<http://www.facebook.com/topic.php?uid=4915599711&topic=54061>`_.  I
+wonder how that happened.
+
+Is symmetric
+------------
+
+Triple equality is still symmetric.
+
+
+Don't use double equality
+-------------------------
+
+Double equality does implicit conversions and besides has some odd
+rules.  My advice is **don't use double equality**.
+
+Triple equality does not do conversions.  If you want to do
+conversions in your comparision my advice is to make them explicit.
+
+
+Compare as string
+-----------------
+
+The easiest conversion is to string.  Here are two immutables, a
+number and a string.
+
+.. code-block:: javascript
+
+   js> a = 0
+   0
+   js> b = '0'
+   0
+
+
+These quantities are double equal but not triple equal.
+
+.. code-block:: javascript
+
+   js> a == b
+   true
+   js> a === b
+   false
+
+Here's how to do an explicit conversion to string before comparison,
+which gives an equality.
+
+.. code-block:: javascript
+
+   js> '' + a === '' + b    // Converts variables to strings.
+   true
+
+.. note::
+
+   It's always easier to read a triple equal comparison, because
+   you're not distracted by the complex double equal rules.
+
+
+Compare as number
+-----------------
+
+[To follow later.]