for(let k of data){}

[baiyukey]

let data=[{a:1,b:2},{a:2,b:3}]; for(let k of data){ //... }

Output error:
.....
at com.yahoo.platform.yui.compressor.YUICompressor$1.runtimeError(YUICompressor.java:172)
at org.mozilla.javascript.Parser.parse(Parser.java:396)
at org.mozilla.javascript.Parser.parse(Parser.java:340)
at com.yahoo.platform.yui.compressor.JavaScriptCompressor.parse(JavaScriptCompressor.java:315)
at com.yahoo.platform.yui.compressor.JavaScriptCompressor.(JavaScriptCompressor.java:536)
at com.yahoo.platform.yui.compressor.YUICompressor.main(YUICompressor.java:147)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.yahoo.platform.yui.compressor.Bootstrap.main(Bootstrap.java:21)

[KingFishTheKing]

I think YUI can't handle 'let', try 'var' instead