javascript - NodeJS 中变量未读取值

Question

这个问题已经有答案了:

Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference (7 个回答)

How do I return the response from an asynchronous call? (42 个回答)

已关闭 3 年前。

我得到的输出为undefined在最后一行代码中。理想情况下，它应该打印 cost 的值我在 call function 的最后一行分配的变量。 cost variable是否正确打印值。我犯了什么错误。或者我如何访问 cost 的值外function .

var request = require("request");
 var itemCost;

 function call() {

   request.get("http://localhost:8080/ords/hr/rest-v3/item/Sugar", (error, response, body) => {

            if (error) {
                return console.dir(error);
            }
            let jsonData = JSON.parse(response.body);
            let obj = new Object(jsonData);

            obj.items.forEach(itemChild => {
                let cost = itemChild.itemcost;
                console.log(cost);
                itemCost = cost;  //Assigning value of cost to itemcost. is this correct way?
            })

        });
    }

    call();
    console.log(itemCost); //here I am getting undefined. I want value of cost here.

Answer 1

因此，request.get是异步，您必须在回调中记录itemCost，否则您需要将您的操作与Promise(这里是 example )或其他东西链接起来。

它是未定义的，因为 console.log(itemCost); 实际上并没有等待 request.get 完成，因此 itemCost 是在到达该线之前不会计算。

Basically I have to access that value in some other function. is there any way to do that?

您只需传递 itemCost 变量即可在回调中调用该函数。例如

request.get("http://localhost:8080/ords/hr/rest-v3/item/Sugar", (error, response, body) => {

    if (error) {
        return console.dir(error);
    }
    let jsonData = JSON.parse(response.body);
    let obj = new Object(jsonData);

    obj.items.forEach(itemChild => {
       let cost = itemChild.itemcost;
       console.log(cost);
       itemCost = cost;  //Assigning value of cost to itemcost. is this correct way?
    })

    foo(itemCost); //call the method you need by passing the calculated variable

     });
    }