How to convert a string into bytes in specific charset (encoding) using JavaScript?

I want to generate text bytes in various charsets (encodings), such as ISO-8859-1, Big5, UTF-8, UTF-16, etc., mostly for testing purposes (i.e. to make sure my app script can correctly handle the provided bytes in these charsets), which is almost like:

new TextEncoder(mycharset).encode(mystring)

Unfortunately TextEncoder only supports conversion of a string into UTF-8 bytes, which is unlike TextDecoder, which supports conversion of bytes in various charset into a string.

The behavior of TextEncoder is defined by the spec and is unlikely going to be changed in the future... Any folk know why there is the inconsistent behavior in both classes? And the way to do the job? (without manually providing a conversion table/map for all the target charsets)

TO REVIEWER: This issue is NOT a duplicate of the related question, which asks why the parameter of TextEncoderdoes not take effect, and the answer says that TextEncoder does not take a parameter by the spec.

This questions specifically asks for:

1. why the spec specified that TextEncoder not take a parameter, which is inconsistent with TextDecoder
2. HOW to perform the conversion as this question asked

Both are not asked and explained in the related question and answer.