Because I always forget how to use these two before , Write them down today , Leave a note for yourself , Also hope to help friends in need .

**

for…in

var arr1 = ["zou", "liu", "chen", "yang", "li", "wu"];
    var arr2 = {
    
      "name": " extra ",
      "age": 24,
      "sex": " male ",
      "birth": "1996-03-06"
    };
    for (let i in arr2) {
    
      console.log(i);
    }
    for (let j in arr1) {
    
      console.log(j);
    }

give the result as follows ：

From this we can see that ,for…in Is to traverse the subscript value of an array or object , It's called key value ,
If you want to use for…in To traverse the values （value） If so ：

var arr1 = ["zou", "liu", "chen", "yang", "li", "wu"];
    var arr2 = {
    
      "name": " extra ",
      "age": 24,
      "sex": " male ",
      "birth": "1996-03-06"
    };
    for (let i in arr2) {
    
      console.log(arr2[i]);
    }
    for (let j in arr1) {
    
      console.log(arr1[j]);
    }

Output ：

What I want to say here is that I can use arr2.name This way to access value value , But in for…in In the loop, you cannot access and obtain the corresponding value value , Upper figure ：

var arr2 = {
    
      "name": " extra ",
      "age": 24,
      "sex": " male ",
      "birth": "1996-03-06"
    };
    for (let i in arr2) {
    
      console.log(arr2.i);
    }

The result is four undefined, The specific reason is not very clear , Welcome friends who know to comment in the comment area below .
summary ： If only to get key value , that for…in Still usable

for…of

var arr1 = ["zou", "liu", "chen", "yang", "li", "wu"];
    var arr2 = {
    
      "name": " extra ",
      "age": 24,
      "sex": " male ",
      "birth": "1996-03-06"
    };
    for (let k of arr1) {
    
      console.log(k);
    }
    for (let l in arr1) {
    
      console.log(l);
    }

result ：

You can see for…of It can be used to traverse objects or arrays value It's worth it .
In addition, by the way forEach() This Method executes the provided function once for each element of the array , On business ： Array .forEach(function( Array of value, Array of key, The array itself ){…})
forEach Use as needed , It's just a mention here