[JS] Support for nullish assignment / class and more ES6 rules

[RickoNoNo3]

Issue

Minify minified a special javascript code file. Then it returned a 0 byte response. After some testings, I found the problem:

With this style, Minify runs well:

args = args ?? [];

With this style, Minify goes wrong:

args ??= [];

more information on the nullish assignment: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment

Version

2.9.13

[RickoNoNo3]

This kind of javascript can not be parsed too:

class A {
  prop = 1;
  constructor() {
    // some code
  };
  method = () => {
    // some code
  };
};

The minifier can't process this kind of class format.

[tdewolff]

Both syntax types are not in the specification but still in the proposal stage. I've added the first as you requested, let me see how much is involved in the class one.

[tdewolff]

It's supported now, however it's still in the proposal phase. Usually the minifiers follow the latest official specification, but this is an exception.