why is "this" undefined in <script type="module">?

Question

Playing around with ECMAScript Modules just found that calling out this in the "global" scope of a script with type module is undefined but in a normal script, it references to the window object

ES Modules script

    <script type="module">
      console.log(this); // undefined
    </script>

Normal script

    <script>
      console.log(this); // window object
    </script>

Is there any spec about this, or can someone explain me why?

Answer 1

ES6 modules are specced such that their this value is undefined. You can see the spec definition in 8.1.1.5.4 Module Environment Records: GetThisBinding().

8.1.1.5.4 GetThisBinding()

1. Return undefined.