闭包编译器和奇怪的预代码

Question

I often use online closure compiler ( https://closure-compiler.appspot.com/home ) to minify my JS code. Now I'm trying to integrate this process into PhpStorm via "External tools" and I'm having very strange result. Everything works fine, except one thing - some strange code is printed before mine if I use Set variables. For example:
Original code :

function a(text) { alert(text); }
$(document).ready(function(){
    let mySet = new Set();
    $("#myButton").click(function(){
       a("CLICKED");
       mySet.clear();
       for(let i=0;i<10;i++) mySet.add(i);
    });
}); 

Minified via WEB-tool :

function a(b){alert(b)}$(document).ready(function(){var b=new Set;$("#myButton").click(function(){a("CLICKED");b.clear();for(var c=0;10>c;c++)b.add(c)})});  

Minified via command line ( java -jar compiler.jar --charset UTF-8 --js closureTest.js --js_output_file closureTest.min.js ): https://pastebin.com/QqGXc6H7

---

Without Set variable:

Original :

function a(text) { alert(text); }
$(document).ready(function(){
    let mySet = [];
    $("#myButton").click(function(){
       a("CLICKED");
       mySet.length = 0;
       for(let i=0;i<10;i++) mySet.push(i);
    });
});

Minified via WEB-tool :

function a(b){alert(b)}$(document).ready(function(){var b=[];$("#myButton").click(function(){a("CLICKED");for(var c=b.length=0;10>c;c++)b.push(c)})});

Minified via command line :

function a(b){alert(b)}$(document).ready(function(){var b=[];$("#myButton").click(function(){a("CLICKED");for(var c=b.length=0;10>c;c++)b.push(c)})});

---

As you can see, WEB-tool compilation works fine, but what's wrong with command line compiling?

Answer 1

Well, I found solution (thanks shiftweave ).
It needed argument --rewrite_polyfills false to work fine.

Answer 2

The compiler by default "transpiles" code from the latest supported language version to EcmaScript 5. To support this it also include the necessary polyfills. If you don't need the polyfills, --rewrite_polyfills false as stated in another answer is one solution but a better one may be to set the --language_out=ECMASCRIPT_2015 or --language_out=ECMASCRIPT_2017 if you are only targetting browsers that support these later language features.

You should set your intended --language_out regardless as the default may change in the future.