=proto
int duk_to_boolean(duk_context *ctx, int index);

=stack
[ ... val! ... ] -> [ ... ToBoolean(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-9.2">ToBoolean()</a>
coerced value.  Returns 1 if the result of the coercion <tt>true</tt>, 0 otherwise.
If <tt>index</tt> is invalid, throws an error.</p>

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

=example
if (duk_to_boolean(ctx, -3)) {
    printf("coerced value is true\n");
}

=tags
stack

=fixme
Describe coercion rules for custom types in a central location and just link
there?