While you might have access to the AST, obfuscated JS is still harder for humans to parse, and reverse engineer what the original function is doing.

For example:

    var _0x11f8=['\x6c\x6f\x67'];(function(_0x417d16,_0x4a5f0f){var _0x2d056f=function(_0x25076e){while(--_0x25076e){_0x417d16['push'](_0x417d16['shift']());}};var _0x26edf0=function(){var _0x522f05={'data':{'key':'cookie','value':'timeout'},'setCookie':function(_0x3d387a,_0x44e067,_0xfea505,_0x41ff72){_0x41ff72=_0x41ff72||{};var _0x47c6cd=_0x44e067+'='+_0xfea505;var _0x39897e=0x0;for(var _0x39897e=0x0,_0x329917=_0x3d387a['length'];_0x39897e<_0x329917;_0x39897e++){var _0x1c67fb=_0x3d387a[_0x39897e];_0x47c6cd+=';\x20'+_0x1c67fb;var _0x4c67f9=_0x3d387a[_0x1c67fb];_0x3d387a['push'](_0x4c67f9);_0x329917=_0x3d387a['length'];if(_0x4c67f9!==!![]){_0x47c6cd+='='+_0x4c67f9;}}_0x41ff72['cookie']=_0x47c6cd;},'removeCookie':function(){return'dev';},'getCookie':function(_0xc34cfd,_0x2e2a50){_0xc34cfd=_0xc34cfd||function(_0x3fc227){return _0x3fc227;};var _0x316ad0=_0xc34cfd(new RegExp('(?:^|;\x20)'+_0x2e2a50['replace'](/([.$?*|{}()[]\/+^])/g,'$1')+'=([^;]*)'));var _0x67e8e8=function(_0x595d8b,_0x29390a){_0x595d8b(++_0x29390a);};_0x67e8e8(_0x2d056f,_0x4a5f0f);return _0x316ad0?decodeURIComponent(_0x316ad0[0x1]):undefined;}};var _0x4341d3=function(){var _0x10361d=new RegExp('\x5cw+\x20*\x5c(\x5c)\x20*{\x5cw+\x20*[\x27|\x22].+[\x27|\x22];?\x20*}');return _0x10361d['test'](_0x522f05['removeCookie']['toString']());};_0x522f05['updateCookie']=_0x4341d3;var _0x44df89='';var _0x4a0249=_0x522f05['updateCookie']();if(!_0x4a0249){_0x522f05['setCookie'](['*'],'counter',0x1);}else if(_0x4a0249){_0x44df89=_0x522f05['getCookie'](null,'counter');}else{_0x522f05['removeCookie']();}};_0x26edf0();}(_0x11f8,0x126));var _0x2135=function(_0x32c6fa,_0x552733){_0x32c6fa=_0x32c6fa-0x0;var _0x18e137=_0x11f8[_0x32c6fa];return _0x18e137;};function _0x33f554(_0x101bb5,_0x53fcac,..._0x27ef18){var _0x36160c=function(){var _0xddce83=!![];return function(_0x3f37b8,_0x278fb0){var _0x3f096b=_0xddce83?function(){if(_0x278fb0){var _0x701868=_0x278fb0['apply'](_0x3f37b8,arguments);_0x278fb0=null;return _0x701868;}}:function(){};_0xddce83=![];return _0x3f096b;};}();var _0x4810be=_0x36160c(this,function(){var _0x31f51a=function(){return'\x64\x65\x76';},_0x28b288=function(){return'\x77\x69\x6e\x64\x6f\x77';};var _0x5848fd=function(){var _0x59a244=new RegExp('\x5c\x77\x2b\x20\x2a\x5c\x28\x5c\x29\x20\x2a\x7b\x5c\x77\x2b\x20\x2a\x5b\x27\x7c\x22\x5d\x2e\x2b\x5b\x27\x7c\x22\x5d\x3b\x3f\x20\x2a\x7d');return!_0x59a244['\x74\x65\x73\x74'](_0x31f51a['\x74\x6f\x53\x74\x72\x69\x6e\x67']());};var _0x46df82=function(){var _0x5b3a8d=new RegExp('\x28\x5c\x5c\x5b\x78\x7c\x75\x5d\x28\x5c\x77\x29\x7b\x32\x2c\x34\x7d\x29\x2b');return _0x5b3a8d['\x74\x65\x73\x74'](_0x28b288['\x74\x6f\x53\x74\x72\x69\x6e\x67']());};var _0x4b23f3=function(_0x5ac3b0){var _0x4153a0=~-0x1>>0x1+0xff%0x0;if(_0x5ac3b0['\x69\x6e\x64\x65\x78\x4f\x66']('\x69'===_0x4153a0)){_0x3d9fe5(_0x5ac3b0);}};var _0x3d9fe5=function(_0x467f75){var _0x4c33bb=~-0x4>>0x1+0xff%0x0;if(_0x467f75['\x69\x6e\x64\x65\x78\x4f\x66']((!![]+'')[0x3])!==_0x4c33bb){_0x4b23f3(_0x467f75);}};if(!_0x5848fd()){if(!_0x46df82()){_0x4b23f3('\x69\x6e\x64\u0435\x78\x4f\x66');}else{_0x4b23f3('\x69\x6e\x64\x65\x78\x4f\x66');}}else{_0x4b23f3('\x69\x6e\x64\u0435\x78\x4f\x66');}});_0x4810be();let _0xdbe94a=_0x101bb5/_0x53fcac;let _0x26ea56=_0x27ef18['\x72\x65\x64\x75\x63\x65']((_0xee94a6,_0x15b1b8)=>_0xee94a6*_0x15b1b8,_0xdbe94a);console[_0x2135('0x0')](_0x26ea56);}_0x33f554(0x1,0x2,0x3,0x4);

and

    function test(x, y, ...args) {
      let z = x / y;
      let ret = args.reduce((a, v) => (a * v), z);
      console.log(ret);
    }
    test(1, 2, 3, 4);

are functionally identical. (I used an off the shelf obfuscator)

No amount of obfuscation will stop a determined reverse engineer from pulling apart your code, but it can increase the cognitive load to the point where most people won't bother.

Also, just in case, nobody should ever run untrusted code, particularly not obfuscated untrusted code. Including the examples I posted above.