SOURCE

function write(string){
    document.write(string+"<br>")
}
var arr = ["HTML", "CSS", "JavaScript", "jQuery", "Vue.js"];
var arr1=arr.slice(1,3)
write(arr)
write(arr1)
arr1.unshift("HTML", "CSS");
write(arr1)
arr.shift();
write(arr)
arr.pop()
write(arr)
var arr = ["HTML", "CSS", "JavaScript", "jQuery", "Vue.js"];
write(arr.sort())

var num=[3,6,9,2,1,7];
write(num.sort())
function upcompare(a,b){
    if(a>b){
        return 1;
    }
    if(a==b){
        return 0;
    }
    if(a<b){
        return -1;
    }
}

write(num.sort(upcompare))

function downcompare(a,b){
    if(a>b){
        return -1;
    }
    if(a==b){
        return 0;
    }
    if(a<b){
        return 1;
    }
}
write(num.sort(downcompare))

var num=[3,6,9,2,1,7];
write(num.reverse())
write(num.join())
write(num.join(":"))

var str1="绿叶学习网"
var str2 = str1.split("").join("><");
 var str2 = str2.split("");
   str2.unshift("<");
   str2.push(">")
write(str2.join(""))