Why is forEach () not working in an iframe in IE11?

This question was closed as off-topic, so I noted the missing information.

Specific problem or bug plus shortest code:

The code below doesn't work in IE11 if it's in the iframe of certain websites. ("Specific" is not specific, but I don't have a public demo. I can't do this until I find the reason in my personal code. However, the question is specific enough to give an expert answer that why I asked SO instead of the lengthy debugging process without any idea.)

['a', 'b'].forEach(function(elem){console.log(elem);});

      

        
        
        
      

    

error says that the array does not support the forEach method.

Desired behavior:

The forEach () method executes the provided function once per array element. - MDN