Lexical declaration cannot appear in a single-statement context

[oljbo]

c# code is:

var jscode = @"
function fun(item){
    if (createmode == true) {
        let a = 'a';
        var b = 'b';
        if(a && b != null ) {
            let Maker = 'c';
            let obj = {a:a,Closed:1,Closer:Maker,CloseDate:new Date()};
            let rejust= null;
        }
    }
}

var createmode = true;
fun(null); 
";
var s1 = Uglify.Js(jscode, new NUglify.JavaScript.CodeSettings() { }).Code;

ths result is: function fun(){if(createmode==!0){let t="a";var n="b";if(t&&n!=null)let n={a:t,Closed:1,Closer:"c",CloseDate:new Date}}}var createmode=!0;fun(null)

when I run the result in brower, It's has error: SyntaxError: Lexical declaration cannot appear in a single-statement context