Question

我在函数中有以下函数，我试图检查对象的状态。问题是，当我运行代码时，它告诉我this.state不是一个对象。

convertNeighbourArrayIntoMap(neighbourData) {

        var neighbourCategoryMap = {}; // Create the blank map

        neighbourData.forEach(function(neighbourItem) {

            if(this.state.searchString == ""){
               console.log("No Search String");
            }

            if (!neighbourCategoryMap[neighbourItem.category]) {
              // Create an entry in the map for the category if it hasn't yet been created
              neighbourCategoryMap[neighbourItem.category] = [];
            }

            neighbourCategoryMap[neighbourItem.category].push(neighbourItem);

        });

        return neighbourCategoryMap;  
    }

这有效，但我需要它在循环内。

convertNeighbourArrayIntoMap(neighbourData) {

        var neighbourCategoryMap = {}; // Create the blank map

if(this.state.searchString == ""){
               console.log("No Search String");
            }

        neighbourData.forEach(function(neighbourItem) {

            if (!neighbourCategoryMap[neighbourItem.category]) {
              // Create an entry in the map for the category if it hasn't yet been created
              neighbourCategoryMap[neighbourItem.category] = [];
            }

            neighbourCategoryMap[neighbourItem.category].push(neighbourItem);

        });

        return neighbourCategoryMap;  
    }

Answer 1

这是因为forEach的回调没有获得相同的上下文。轻松修复它切换到箭头功能（如果可能）：

       neighbourData.forEach((neighbourItem) => {

            if(this.state.searchString == ""){
               console.log("No Search String");
            }

            if (!neighbourCategoryMap[neighbourItem.category]) {
              // Create an entry in the map for the category if it hasn't yet been created
              neighbourCategoryMap[neighbourItem.category] = [];
            }

            neighbourCategoryMap[neighbourItem.category].push(neighbourItem);

        });

如果您没有可用的箭头功能，那么bind forEach功能：

       neighbourData.forEach(function(neighbourItem) {

            if(this.state.searchString == ""){
               console.log("No Search String");
            }

            if (!neighbourCategoryMap[neighbourItem.category]) {
              // Create an entry in the map for the category if it hasn't yet been created
              neighbourCategoryMap[neighbourItem.category] = [];
            }

            neighbourCategoryMap[neighbourItem.category].push(neighbourItem);

        }.bind(this));

Answer 2

使用箭头功能按预期保留this上下文：

neighbourData.forEach((neighbourItem) => {
...
  if (!this.state.searchString.length) {
    console.log("No Search String");
  }
...

在循环中，this.state不是对象

2 个答案: