"use strict" is not respected in theme header

If put in header.html in a theme component, the following code does not generate an error which is should according to https://www.w3schools.com/js/js_strict.asp:

<script type="text/discourse-plugin" version="0.8">
"use strict";
x = 3.14;       // This should cause an error because x is not declared

I’d prefer if strict was enabled by default, but that’s another story.