第九天：数组排排序
<h5>给object数组进行排序（排序条件是每个元素对象的属性个数）</h5>
<pre><code>const ObjectArray = [
{a:1,b:2,c:3,d:4},
{a:1,b:2},
{a:2,b:3,c:4}
]
// ps 没说升序降序
console.log(ObjectArray.sort((a, b) => Object.keys(b).length - Object.keys(a).length));
</code></pre>
<h5>实现如下语法的功能：var a = (5).plus(3).minus(6); //2</h5>
<pre><code>Number.prototype.plus = function(n) { return this + n }
Number.prototype.minus = function(n) { return this - n }
console.log((5).plus(3).minus(6));</code></pre>
<h5>实现如下语法的功能：var a = add(2)(3)(4); //9</h5>
<pre><code>const add = (x, i = 0) => i+1 == 3 ? x : (n) => add(x+n, i+1)
console.log(add(2)(3)(4));</code></pre>