1. Deep copy 2. Call apply bind 3. For of in differences

1、Json.parse(Json.Stringfy()） Unable to implement functions and undefined Deep copy , How to solve it ？
Let's write a deep copy

  function deepClone(target) {
      if (target instanceof Object) {
        let dist
        if (target instanceof Array) {
          dist = []
        } else if (target instanceof Function) {
          dist = function () {
            return target.call(this, ...arguments)
          }
        } else {
          dist = {}
        }
        for (let key in target) {
          //  Filter out the attributes on the prototype 
          if (target.hasOwnProperty(key)) {
            dist[key] = deepClone(target[key])
          }
        }
        return dist
      } else {
        return target
      }
    }

2、call apply bind Use of methods

    var obj1 = {
      a: 2,
      b: 3,
      say: function (x, y) {
        return this.a + x + y
      },
    }
    var obj2 = {
      a: 3,
      b: 4,
    }
    var s1 = obj1.say.call(obj2, 5, 6)
    var s2 = obj1.say.apply(obj2, [7, 8])
    var s3 = obj1.say.bind(obj2)
    console.log(s1)
    console.log(s2)
    console.log(s3(1, 2))

    function f1(x, y) {
      console.log(x + y, this)
    }
    f1(10, 20) //30 undefined

    f1.call() //NaN undefined
    f1.apply() //NaN undefined

    f1.call(null, 2, 3) //5 null
    f1.apply(null, [2, 3]) //5 null

    var f2 = f1.bind(null)
    f2(2, 3) //5 null

    var obj = {
      age: 15,
      name: 'lalal',
    }
    f1.call(obj, 2, 3) //5 {age: 15, name: 'lalal'}
    f1.apply(obj, [2, 3]) //{age: 15, name: 'lalal'}
    var f2 = f1.bind(obj)
    f2(2, 3) //5 {age: 15, name: 'lalal'}

    var person = {
  name: ' Zhang San ',
  describe: function () {
    return ' full name ：'+ this.name;
  }
};

console.log(person.describe());// full name ： Zhang San 
var A = {
  name: ' Zhang San ',
  describe: function () {
    return ' full name ：'+ this.name;
  }
};

var B = {
  name: ' Li Si '
};

B.describe = A.describe;
console.log(B.describe());
// " full name ： Li Si "
function f() {
  return ' full name ：'+ this.name;
}

var A = {
  name: ' Zhang San ',
  describe: f
};

var B = {
  name: ' Li Si ',
  describe: f
};
console.log(A.describe()); // " full name ： Zhang San "
console.log(B.describe()); // " full name ： Li Si "

var obj1 = {
      name: 'zhangjing',
      aaa: function () {
        console.log(this.name)
      },
      bbb: function () {
        setTimeout(
          function () {
            this.aaa()
          }.apply(obj1),
          3000
        )
      },
    }
    obj1.bbb()//zhangjing

3、for of and for in difference

1、 Circular array
Difference one ：for in and for of Circular arrays are OK ,for in The output is the of the array index Subscript , and for of The output is the value of each item of the array .

  const arr = ["orange", "apple", "banana", "pear"];
    for (let key of arr) {
      console.log(key); // orange  apple banana pear
    }
    for (let key in arr) {
      console.log(key); //0 1 2 3
    }
    for (const [key, value] of arr.entries()) {
      console.log(key, value); //0 'orange' 1 'apple' 2 'banana' 3 'pear'
    }

2、 Loop objects
Difference two ：for in You can traverse objects ,for of Can't traverse object , Can only traverse with iterator Interface , for example Set,Map,String,Array

   const obj = { 1: "orange", 2: "apple", 3: "banana", 4: "pear" };
    for (let key of obj){
      console.log(key);// Error in created hook: "TypeError: [object Object] is not iterable!"

    }
    for (let key in obj) {
      console.log(key); // 1 2 3 4
      console.log(obj[key]); // orange  apple banana pear
    }

3. Array objects

  const fruits = [
      { 1: "orange" },
      { 2: "apple" },
      { 3: "banana" },
      { 4: "pear" },
    ];

    for (let value of fruits) {
      console.log(value); //{1: 'orange'}  {2: 'apple'}  {3: 'banana'} {4: 'pear'}
      for (let key in value) {
        console.log(key, value[key]);//1 orange  2 apple 3 banana 4 pear
      }
    }

summary ：for in Suitable for traversing objects ,for of Suitable for traversing arrays .for in Traversing is the index of the array , Object properties , And the attributes on the prototype chain .