TypeError: Cannot read properties of undefined (reading 'length') in nodejs

Question

我正在尝试制作一个不和谐级别的机器人，我需要从 json 文件中获取一些 ingo 并比较长度，但我在 if 语句的标题中得到错误：

if(message.author.bot == false && userinput != '!level')
    {   let data = JSON.parse(fs.readFileSync("./level.json", "utf-8"));
        // console.log(data);
        if(data = undefined)
        {
            console.log("data is undefined");
            return;
            //if date is undefined (failsafe method)
        }
        // for loop looping through array, if we are going to find user, we add +1 experience and exit the loop
        if( data.length > 0){
        for(let i=0;i< data.length; i++)
        if(message.author.id == data[i].userID)
        {
            data[i].exp++;
            fs.writeFileSync("./level.json", JSON.stringify(data));
            i = data.length;
        }
            
        }
        //if file is empty, add user details to file, only run once
        else
        if(data.length <= 0)
        {
        const newuser = {
                    "userID" : message.author.id,
                    "exp" : 1
                }
                data = [newuser];
                fs.writeFileSync("./level.json", JSON.stringify(data));
        }
        
        //is going to add experience to user
        
    }

错误日志：

    if( data.length > 0){
             ^

TypeError: 无法读取未定义的属性（读取 'length'）在客户端。<匿名>

Answer 1

您正在分配（单等号）undefined给数据：

if(data = undefined)
        ^^^

如果你检查 double equals，它会起作用：

if (data == undefined) { ... }

你也可以if (!data) { ... }.

Answer 2

这是因为if(data = undefined)。请注意，只有一个 = 符号。

因此，data 将被分配为 undefined，此行将变为 if(undefined)。因此 if 块将不会被执行。

只需将此行更新为 if(data == undefined)

TypeError: Cannot read properties of undefined (reading 'length') in nodejs

TypeError: Cannot read properties of undefined (reading 'length') in nodejs

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