Vue.js and TypeScript combination may result in a 'null' value when using file input

Question

Vue.js and TypeScript combination may result in a 'null' value when using file input

I am attempting to detect an event once a file has been uploaded using a file input.

Here is the JavaScript code:

fileSelected(e: Event) {
    if ((<HTMLInputElement>e.target).files !== null && (<HTMLInputElement>e.target).files[0] !== null) {
        this.file = (<HTMLInputElement>e.target).files[0];
    }
}

My file variable is currently set to null. I am trying to access the first element of the files array, but it is throwing an error:

Object is possibly null

I have discovered that you need to verify if it's not empty first, but I still encounter the same error on the if statement itself. Any suggestions?

javascript typescript vue.js vuejs2

Answer 1

Answer №1

In the provided code snippet, TypeScript is unable to narrow down types which results in errors. To resolve this issue, you can (1) store the values in variables and (2) implement guard clauses to ensure non-null value checks:

selectFile(e: Event) {
  const input = e.target as HTMLInputElement;
  const fileList = input.files;

  // Guard clause to handle null or falsy values for `fileList` and `fileList[0]`
  if (!fileList || !fileList[0])
    return;

  // After these checks, both `fileList` and `fileList[0]` will have non-null values
  // Additionally, `fileList` will be inferred as type `FilesList` instead of `FilesList | null`
  this.selectedFile = fileList[0];
}

Answer 2

In the provided code snippet, TypeScript is unable to narrow down types which results in errors. To resolve this issue, you can (1) store the values in variables and (2) implement guard clauses to ensure non-null value checks:

selectFile(e: Event) {
  const input = e.target as HTMLInputElement;
  const fileList = input.files;

  // Guard clause to handle null or falsy values for `fileList` and `fileList[0]`
  if (!fileList || !fileList[0])
    return;

  // After these checks, both `fileList` and `fileList[0]` will have non-null values
  // Additionally, `fileList` will be inferred as type `FilesList` instead of `FilesList | null`
  this.selectedFile = fileList[0];
}

Vue.js and TypeScript combination may result in a 'null' value when using file input

Answer №1

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