高阶函数:
	所谓高阶函数，就是函数的参数可以是函数
	map
	filter
	reduce
<script>
    function fn(n){
        return Math.abs(n);
    }
    function add(x,y,fn){ //高阶函数
        return fn(x)+fn(y);
    }
    let res = add(-10,10,fn);
    //20
</script>

<script>
    function add(x,y,fn){ //高阶函数
        return fn(x)+fn(y);
    }
    let res = add(-100,10,Math.abs);
    //110
</script>

<script>
    function fn(n){
        return n**2
    }
    let arr = [1,2,3];
    let res = arr.map(fn);
    // [1, 4, 9]
</script>

<script>
    let arr = [1.3,2.5,3.9,9.1];
    let res = arr.map(Math.round);
    //[1, 3, 4, 9]
</script>

<script>
    let arr = [1.3,2.5,3.9,9.1];
    let res = arr.map(String);
    //["1.3", "2.5", "3.9", "9.1"]
</script>

<script>
    let arr = [1,2,3,4,5];
    let res = arr.reduce((pre,cur)=>pre+cur,10);
    //25
</script>