Javascript：typeof显示“对象”，然后抛出空指针

Question

Maybe someone can shed light on this.

I'm getting JS's version of an NPE on occasion, even though logging shows that typeof the offending variable is 'object'.

Here's the logging:

typeof myVar: object
ERROR in main._getFolderCount(): TypeError: myVar is null
console.error("ERROR in main._getFolderCount(): " + e); 

Here's the code:

try{
  console.log('typeof myVar: ' + typeof myVar);
  if (typeof myVar !== 'undefined' && typeof myVar !== 'null'){
    if (currentMsgsObj && currentMsgsObj.folderId == data[i].id && myVar.totalRows!=data[i].count) {        
      myVar.totalRows=data[i].count;
    }
  } else { 
  }
  } catch (e) {
    console.error("ERROR in main._getFolderCount(): " + e);
  }
}

So you can see that the logging shows myVar is typeof 'object', and the code appears to pass the 'undefined/null' check, and then proceeds to blow up when a myVar member is accessed.

Answer 1

该null类型只具备返回object使用时typeof在其运营商。

Answer 2

In JavaScript, typeof null == 'object' .

You can check whether something is null using === :

if (blarg === null) ...

Using typeof blarg == 'null' will never work.