转换成字符串类型
通过与空字符串相加，可以将某个变量快速转换成字符串类型。
两种for循环
for (let value of array) {
// do something with value
}
for (let property in object) {
// do something with object property 用对象属性做某事
}
剩余参数语法允许我们将一个不定数量的参数表示为一个数组。
function sum(...theArgs) { return theArgs.reduce((previous, current) => { return previous + current; }); }
console.log(sum(1, 2, 3)); // expected output: 6
console.log(sum(1, 2, 3, 4)); // expected output: 10
