JavaScript - 嵌套forEach中的变量循环到undefined

Question

我试图在第二个for循环中获取this.userId的值，它通过undefined。 看看下面的代码：

 //the following variable gets undefined inside the second for loop
  this.userId = localStorage.getItem('userId');

  this.reviews.forEach(function(element1){

    element1.usersWhoLike.forEach(function(element2) {

   if(element2 == this.userId){
       //this throughs undefined !!
       console.log('UserID is : ',this.userId)
   }

  });

  });

Answer 1

你错过了forEach里面的功能范围 我们试试吧：

this.reviews.forEach(function(element1){

    element1.usersWhoLike.forEach(function(element2) {

   if(element2 == userId){
       //this throughs undefined !!
       console.log('UserID is : ',this.userId)
   }

  }, this);

  }, this);

Answer 2

this.userId在第二个中指的是函数范围本身。尝试添加var self作为参考

    this.userId = localStorage.getItem('userId');

    var self = this; // Referencing
    this.reviews.forEach(function(element1){

        element1.usersWhoLike.forEach(function(element2) {

            if(element2 == userId){
                //this throughs undefined !!
                console.log('UserID is : ',self.userId)
            }

        });

    });

Answer 3

this.userId = localStorage.getItem('userId');

将此行置于第一个循环

中