What could be causing the "ERROR TypeError: Cannot read property 'length' of undefined" message to occur with a defined array in my code?

Even though I defined and initialized my array twice, I am encountering a runtime error: "ERROR TypeError: Cannot read property 'length' of undefined."

I have double-checked the definition of the array in my code, but Angular seems to be playing tricks on me. I have attempted various solutions such as changing the syntax to "filesToUpload && filesToUpload?.length > 0," yet Angular is still not functioning correctly in my opinion.

Here is an excerpt from the component.html file:

<ng-container *ngIf="undefined !== filesToUpload && filesToUpload.length > 0">
  <button (click)='uploadImages()' 
           class="btn btn-success" 
           type="button">
          <i class="fas fa-upload"></i>Upload
  </button>

  <button (click)='cancelUpdate()' 
          class="btn btn-danger" 
          type="button">Cancel</button>
</ng-container>

The corresponding component.ts file looks like this:

export class ImageGalleryUploadComponent implements OnInit 
{
  filesToUpload: File[] = [];
  ngOnInit() {
    this.filesToUpload = []
  }
}

Could this be an issue with Angular itself or possibly related to the component lifecycle?

Answer №1

Check this out, it seems like you may be mistakenly assigning undefined to your Array object.

<ng-container *ngIf="filesToUpload && filesToUpload != 'undefined' && filesToUpload.length > 0">

Although not recommended, give it a try once and if the issue persists, double-check your code for any such assignments.

Answer №2

To efficiently handle both null values and file lengths at once, consider implementing this method using the safe navigation operator:

<ng-container *ngIf="filesToUpload && filesToUpload?.length > 0">

Answer №3

Simply execute it once like this:

export class ImageGalleryUploadComponent implements OnInit 
{
  filesToUpload: File[] = [];
}

I have encountered the same problem before. Removing the re-initialization in ngOnInit should resolve the issue. And remember, every time the component is initialized, it will be set to an empty array [].

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