=proto
double duk_to_number(duk_context *ctx, int index);

=stack
[ ... val! ... ] -> [ ... ToNumber(val)! ... ]

=summary
<p>Replace the value at <tt>index</tt> with an Ecmascript
<a href="http://www.ecma-international.org/ecma-262/5.1/#sec-9.3">ToNumber()</a>
coerced value.  Returns the coerced value.  If <tt>index</tt> is invalid, throws
an error.</p>

<p>Custom type coercion:</p>
<ul>
<li>Buffer: <tt>0</tt> for zero-size buffer, <tt>1</tt> otherwise</li>
<li>Pointer: <tt>0</tt> for <tt>NULL</tt> pointer, <tt>1</tt> otherwise</li>
</ul>

=example
printf("coerced number: %lf\n", duk_to_number(ctx, -3));

=tags
stack

=fixme
Are the current custom coercions sensible?  For instance, should pointer
coerce to its numeric value?