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

=stack
[ ... val! ... ]

=summary
<p>Validate argument index, return 1 if index is valid, 0 otherwise.</p>

=example
if (duk_is_valid_index(ctx, -3)) {
    printf("index -3 is valid\n");
} else {
    printf("index -3 is not valid\n");
}