vue watch error: Error in callback for watcher "xxx": "TypeError: Cannot read properties of undefined ...

Table of contents

1. Problem

2. Solution

3. Summary

---

1. Problem

1. After listening to an object in data, it actually reported an error. The error is strange: you cannot use the apply method on undefined

1) The detailed errors are as follows:

?9b69:5059 [Vue warn]: Error in callback for watcher "flowCategory": "TypeError: Cannot read properties of undefined (reading 'apply')"

found in

---> <FlowItem> at src/projects/comen/implemenceWorkstation/flowSetting/
       <ElTabPane> at packages/tabs/src/
         <ElTabs> at packages/tabs/src/
           <View> at src/views/implemenceWorkstation/flowSetting/
             <Index> at src/layout/
               <App> at src/
                 <Root>
warn$2 @ ?9b69:5059
logError @ ?9b69:3728
globalHandleError @ ?9b69:3724
handleError @ ?9b69:3691
invokeWithErrorHandling @ ?9b6

2) The code is as follows:

  watch: {
    flowCategory: {
      hanlder(newval, oldval) {
        ('flowCategory', newval, oldval);
      },
      deep: true
    }
  }

2. Solution

1. Is it okay to see the above code? I am also very confident. Isn’t it just that the values ​​before and after the change were printed in the handler function? This is how I write it, and I am so confident.

2. I found it after searching for a long timeI wrote the word handler wrongly, if you type incorrectly, it was written as 'hanlder', there was no prompt for ide &***&

Change it to the following

  watch: {
    flowCategory: {
      handler(newval, oldval) {
        ('flowCategory', newval, oldval);
      },
      deep: true
    }
  }

3. I finally understand nowError: It is to tell us that there is a problem with the handler. Watch doesn't know what to do when listening to changes.。

3. Summary

1.Report the above error, which means there is a problem with the handler function.. You can check the following points:

  1) Is the handler function written?

  2) Is the handler spelled correctly (Although it's a bit stupid, I'm really because of this today...)

   3) The handler function cannot be an arrow function(After this pointing changes, apply is also problematic)

2. Ah, stupid me.

/*

Hope it helps you!

If there are any errors, please correct me! Thank you so much!

*/