console.log(Number.isSafeInteger(10)); //trueconsole.log(Number.isSafeInteger(10.4)); //false

  console.log(Math.trunc(4.11)); //4console.log(Math.trunc(4.9)); //4

console.log([1, 2, 3, 4, 5].copyWithin(0, 3, 4)); //[4, 2, 3, 4, 5]

console.log([1, 2, 3, 4, 5, 6].findIndex(item => item > 3)); //3

  console.log([1, 2, NaN].includes(1)); //trueconsole.log([1, 2, NaN].includes(NaN)); //true

参数

  function test(x, y = "world") {console.log("默认值", x, y);}test("hello"); //hello world

  let x = "test";function test2(x, y = x) {console.log("作用域", x, y);}test2("kaka"); // kaka kaka

  let x = "test";function test2(c, y = x) {console.log("作用域", c, y);}test2("kaka"); // kaka test

rest 参数, 扩展运算符

  // 把一些离散的值转换成了数组function test3(...arg) {for (let v of arg) {console.log("rest", v);// 1// 5// a
    }}test3(1, 5, "a");// 把数组转成离散的值console.log("a", ...[1, 2, 4]); //a 1 2 4

箭头函数

  // this 绑定// es5: 函数被调用时的对象// es6: 定义时候的对象let arrow = v => v * 2;console.log(arrow(3)); //6let arrow2 = () => 5;console.log(arrow2()); //5

尾调用

  function tail(x) {console.log("tail", x);}function fx(x) {return tail(x);}fx(123); // 123

简洁表达

  let o = 1;let k = 2;let es5 = { o: o, k: k };let es6 = { o, k };console.log(es5);console.log(es6);// {o: 1, k: 2}
let es5_method = {hello: function() {console.log("hello es5");}};let es6_method = {hello() {console.log("hello es6");}};es5_method.hello();es6_method.hello();

属性表达式

  let a = "b";let es5_obj = {a: "c"};// 中括号包起变量，a 是变量，变量的值是blet es6_obj = {[a]: "c"};console.log(es5_obj); //{a: "c"}console.log(es6_obj); //{b: "c"}

新增api

  // 判断相等，等同 ===console.log(Object.is("abc", "abc")); //true// 引用类型 引用两个不同地址console.log(Object.is([], [])); //false// 浅拷贝 属性有限制 ，拷贝自身属性的地址console.log("拷贝", Object.assign({ a: "a" }, { b: "b" })); //{a: "a", b: "b"
let test = { k: 123, o: 456 };for (let [key, value] of Object.entries(test)) {console.log([key, value]);// ["k", 123]  // ["o", 456]

扩展运算符 (es7特性, 支持度不高)

  //扩展运算符 (es7特性)//let { a, b, ...c } = { a: "test", b: "kill", c: "ddd", d: "ccc" };// c= {//   c:ddd,//   d:ddd// }

9. symbol 类型

  // 同键值不会造成冲突let a1 = Symbol.for("abc");let obj = {[a1]: "123",abc: 345,c: 456};console.log(obj); //{abc: 345, c: 456, Symbol(abc): "123"}/*** 对象中有用到symbol作key值时，for in 和 let of 是拿不到属性的*/for (let [key, value] of Object.entries(obj)) {console.log(key, value);// abc 345// c 456
  }for (let key in obj) {console.log(key, obj[key]);// abc 345// c 456
  }// 这时候就需要一个api了,得到一个数组Object.getOwnPropertySymbols(obj).forEach(item => {console.log(obj[item]);// 123 (只取到symbol)
  });// 所有都要那到要使用Reflect, 返回symbol变量作为key值的属性Reflect.ownKeys(obj).forEach(item => {console.log(item, obj[item]);// abc 345// c 456// Symbol(abc) '123'});