GX4005: call is not allowed in a client expression

Cause#

A client expression calls a function that has no JavaScript form. A client expression can call the functions of package gxc only.

Example#

package cart

import "strings"

signals {
  Name string = ""
}

<p show={strings.Contains($Name, "a")}>Has an a</p>

gx check reports:

cart/Cart.gx:9:10: GX4005: only gxc helpers are allowed in a client expression

Fix#

1. Use a gxc function when one does the work.
2. Or calculate the value on the server and pass it as a prop.